In a small bowl, combine all seasoning sauce ingredients and mix well.
Slice the tofu into bite-sized cubes. Cut the onion into thin half-moon slices. Chop the green onion into 3-4 cm pieces. Slice the mushrooms into bite-sized pieces.
Place the sliced onion at the bottom of a pot. Add 1 anchovy tablet and 1½ cups of water (or rice-washed water).
Layer the tofu, mushrooms, and green onion on top. Pour the seasoning sauce over everything.
Turn the heat to high and bring the pot to a boil. While boiling, scoop broth from the bottom and pour it over the tofu to help the sauce mix evenly.
Once boiling, reduce the heat to medium and simmer for 20 minutes, stirring occasionally.
When the sauce is well-absorbed and slightly thickened, transfer to a serving dish. Enjoy with steamed rice and banchan (Korean side dishes)!