Coat tofu with cornstarch in a bowl. Heat 2 tablespoons vegetable oil in a wok or large skillet over medium heat; fry tofu until lightly browned on all sides, 1 to 2 minutes per side.
Place noodles in a bowl and pour in enough boiling water to cover; soak until softened, about 3 minutes. Drain.
Combine water, sriracha, soy sauce, sugar, tamarind concentrate, and red pepper flakes in a saucepan over low heat. Cook and stir sauce until flavors blend, about 5 minutes.
Heat 1 tablespoon vegetable oil in a wok over medium-high heat. Add tofu, noodles, and sliced onion; cook and stir until tofu is golden brown; about 3 minutes. Stir in sauce gradually until noodles are well-coated.
Push noodle mixture to one side of the wok. Crack egg onto opposite side of wok; stir until beginning to set, 30 seconds to 1 minute. Stir egg into noodles gently. Garnish with green onions, peanuts, and lime wedges.