MOUNTAIN VIEW'S Xanh restaurant epitomizes the new breed of upscale, contemporary Vietnamese restaurants popping up across Silicon Valley. Although few in number relative to the many low-rent pho shops and banh mi joints, these modern Vietnamese restaurants are developing into a restaurant category all their own. Borrowing techniques and ingredients from the West while showcasing traditional Vietnamese dishes in stylish dining rooms backed by cool beats and lively bar scenes, these places are pushing boundaries like no other "ethnic" restaurants in the South Bay.

While the fusion of East and West doesn't always make sense on the plate, these restaurants represent some of Silicon Valley's most dynamic dining. Culinary traditions are being rewritten. Rules are being broken. Which is only natural given Vietnamese food's receptivity to other food cultures.

Even before Vietnamese food came to America on the wave of immigration in the early 1980s, the cuisine had already borrowed liberally from other culinary traditions like China (noodles, bean curd), France (baguettes, mayonnaise, coffee) and India (curries). So it's in keeping with Vietnamese food's open-door policy to continue to fuse elements of other cuisines into its repertoire. In the case of Xanh, that means ingredients like green apple, smoked salmon, chocolate and even Parmesan cheese.

In spite of some slip-ups, Xanh (pronounced "zahn") shows promise. Xanh opened two years ago, but moved across restaurant-packed Castro Street two months ago into a bigger space that boasts a full bar and lounge that fronts sexy, multichambered dining rooms lit by soft green and blue lights. It's a great-looking place that feels more like stepping into a club than a restaurant.

The menu, which ranges from the traditional to the unconventional, has been upgraded as well. There are a host of rolls, several salads, lots of noodle dishes, small plates and full-size entrees. And there's plenty to recommend.

One of my favorites dishes is the "ankle biters" ($19), eight shell-on stir-fried prawns wearing a salty, spicy crust and tossed with garlic, onion, chiles and lemon leaves. Although some of the vegetables were staggeringly salty, the flavor of the piquant spices combined with the juicy, peel-and-eat crustaceans were great.

Bun thit nuong ($12) is a classic that gets an update here. Available with beef, pork, chicken, shrimp or catfish, the dish of tangy rice noodles topped with fresh herbs and vegetables is tossed tableside. While the noodles themselves were nothing special, the lemon-grass-spiked pork was wonderfully charred and juicy and loaded with bright, aromatic flavors.

I also loved the "crispy shrimp clouds" ($10–$12), little light-as-a-cotton-ball rice flour muffins filled with diced shrimp, tart sliced green apples and mint and served with nuoc cham, a sweet-and-sour fish sauce–based vinaigrette. It's a beautiful and delicious dish that's perfect for sharing.

Another great riff on a classic is the "full moon wraps" ($14). Unlike the big, turmeric-infused rice flour crepes called banh xeo from which the dish gets its inspiration, Xanh's are served open-faced and not much bigger than a silver dollar. They're topped with shrimp, bean sprouts, sliced mango, sliced onions and a tangle of fresh herbs. The crepes are set atop dewy leaves of butter lettuce and the idea is to pop the whole thing in your mouth. And it's a good idea, too. With a dribble of nuoc cham or peanut sauce, the whole package bursts with sweet, salty, sour and spicy flavors all at once.

And don't overlook the eggplant ($8), a side dish that combines delicately steamed eggplant with ground chicken, shrimp flakes, peanuts and green onions. The long and skinny dish it's served on is a sight to behold, too.

Spring rolls are one of the most recognizable and beloved of Vietnamese foods. The fresh, brightly flavored rice-paper rolls are like hand-held salads and are typically stuffed with pork and/or shrimp. Xanh takes them into new territory with a long list of eclectic rolls that include things like ahi and mango, catfish and pineapple, soft-shell crab and marinated beef. To establish a baseline, I started with the "traditional roll" ($9), a combination of poached shrimp, pork, rice noodles, cilantro, mint and sliced green apple, a not-so-traditional ingredient. Dipped into the sweetish nuoc cham, it's clean and lively tasting but not much different from what you'd find at a humble mom-and-pop Vietnamese restaurant for half the cost. You're better off going for the more exotic rolls, such as the "rolling duck" ($10), a meaty roll made with Peking-style duck and mango.

Green papaya salad ($10) is another Vietnamese standard, but unfortunately it's substandard here. Loaded with strands of papaya, carrots, sliced mango, cilantro, mint and fat-poached shrimps, I thought it would be bursting with flavor, but it was strangely dull. The shrimp were fresh and sweet, but dousing the salad with nuoc cham failed to perk it up.

Pho ($10) is probably the best-known Vietnamese dish, but it falls short here. The broth of the beef noodle soup is oddly flat and lacks the multilayered flavors of star anise and slow-simmered beef bones that make this soup so good. The braised sirloin in the soup was mushy, too.

Peppercorn beef ($18) fully crossed the line from East to West. Thin slices of tender filet mignon are served with a mild, faintly sweet gravylike peppercorn sauce and served with a jumble of potatoes, asparagus and red and green bell peppers. It's good, but seems more appropriate for a Hungry Hunter than an upscale Vietnamese restaurant.

Desserts are the weakest part of the menu. The multilayered hazelnut mousse, served under a glass dome that's removed with a flourish, is decent but not as spectacular as the presentation suggests. The "8th wonder" ($8), a pyramid of chocolate mousse dusted with cocoa powder with a center of caramel, is pretty good. Skip the "ménage a trios" ($8), a visually striking, deconstructed version of a classic dessert that combines tapioca pearls, sweet mung beans and iced coconut milk. The trio of desserts looked cool, but the gummy tapioca and chewy mung beans were redundant and hard to eat.

Servers, clad in black and wearing bright silver "Xanh" belt buckles, are generally friendly and efficient, but the staff is still feeling its way. On one visit an entree went missing. Then our wine failed to show up. While we were waiting for the entree, we were informed the kitchen had closed. After a long wait, food and wine were delivered with apologies and the entree and desserts generously removed from the bill.

Through it all though, Xanh is still an exciting place to eat. For newcomers to Vietnamese food it's a less than traditional introduction to Vietnamese food but still solidly Vietnamese in spirit and execution. For those already versed in Vietnamese food, Xanh offers the chance to witness culinary boundary crossing with results that are often delicious.