Add enough vegetable oil to heavy medium saucepan to reach depth of 2 inches. Heat vegetable oil over medium-high heat to 350°F. Deep-fry tofu squares in 2 batches until golden, about 2 minutes. Using slotted spoon, transfer tofu to paper towels to drain. Sprinkle with salt and pepper.
Heat 3 tablespoons vegetable oil in heavy large skillet over medium heat. Add minced lemongrass, minced garlic, and chopped jalapeño; sauté until fragrant, about 1 minute. Add sliced onion, red bell pepper, and sliced shiitake mushrooms; sauté until vegetables are crisp-tender, about 3 minutes. Add fish sauce and sugar; sauté 1 minute. Add fried tofu squares and sauté until heated through, about 1 minute. Transfer to bowl. Sprinkle with chopped fresh cilantro and serve.
Fish sauce (nuoc mam) is available at Asian markets and in the Asian foods section of many supermarkets.
