Pour off the oil from the frying pan leaving only about two tablespoonfuls. Reheat the oil.
Over medium heat, cook the quartered mushrooms with the sliced onion in the hot oil with a little salt and pepper for about two minutes.
Add the tofu back to the pan. Stir.
Take about a quarter cup of the sauce, pour over the tofu and mushrooms and cook, tossing, for about half a minute to heat the tofu cubes through.
Spoon the mushrooms and tofu with lemongrass chili sauce into small bowls. Drizzle any remaining sauce in the pan directly over the mushrooms and tofu. Sprinkle with sliced scallions before serving.