Heat wok with the remaining 1/4 cup peanut oil, and stir-fry onion and ginger for 30 seconds on medium-high heat. Add peppers, oyster mushrooms, shiitake mushrooms, and stir-fry for another minute. Add soy sauce and oyster sauce; stir-fry for 1 minute more. Add the tofu, and season with salt to taste. Garnish wit scallions.