Soak kabocha in a bowl of water for 15 minutes. Drain.
Combine kabocha squash, dashi stock, fried tofu, onion, sake, mirin, soy sauce, sugar, and salt in a saucepan. Cover with aluminum foil. Bring to a boil; reduce heat and simmer until kabocha squash is tender, about 15 minutes. Remove aluminum foil carefully.
Cool before serving, about 10 minutes.
