This dish is unbelievably easy and the only ingredients you need are just dashi kombu and tofu. 

Hot Tofu (Yudofu)

INGREDIENTS

• 1 medium firm tofu (in the US it’s called “regular” tofu, which is between silken and firm, momen tofu)
• Mitsuba (Japanese parsley) (or any green vegetable you like for color)
• 2 inch kombu (dried kelp)
• 1 Tbsp sake
• ¼ tsp salt (kosher or sea salt; use half if using table salt)

Sauce (optional):

• ¼ cup soy sauce
• 1 Tbsp sake
• 1 tsp mirin
• ½ cup katsuobushi (dried bonito flakes) (Pack loosely; 3 g or 1 pack)

Toppings:

• Green onion/scallion (Finely chopped)
• ginger (grated,optional)
• Shichimi Togarashi (Japanese seven spice) (optional)
• Yuzu kosho (optional)

INSTRUCTIONS

1. Gather all the ingredients.
2. Remove any dust or particles on kombu with a damp cloth (Never wash under running water!) but leave the white powdery substances which contribute to the umami flavor in the stock.
3. In donabe (Japanese clay pot), put dashi kombu and pour water till 70% full. Let the kombu soak for 1-2 hours. Or you can soak the kombu while cooking on the lowest heat for 15 minutes if you are in a hurry.
4. Meanwhile, make the sauce. Put sauce ingredients in a small sauce pan.
5. When boiling, add bonito flakes (katsuobushi).
6. Mix well with chopsticks and cook for 1-2 minutes.
7. Pour the sauce into a small serving dish. You don’t have to strain; it’s okay if some bonito flakes go into the sauce.
8. These leftover bonito flakes have a nice flavor and you can use it as filling for Okaka Onigiri.
9. When the kombu dashi stock is ready (after 1-2 hours), bring the stock to a boil on medium-high heat. Once boiling, add sake and salt.
10. When boiling again, cut the tofu to any size you like (whole tofu, halves, 6 pieces, etc) and put it in the kombu dashi stock.
11. When tofu is heated through, add mitsuba. If you plan to use other vegetables, make sure are they cooked thoroughly.
12. Serve with the sauce and toppings of your choice while the tofu is hot.

https://www.justonecookbook.com/hot-tofu-yudofu/