Slice the onion, carrot, and garlic thinly. Prepare the mackerel fillets by removing any bones.

Heat olive oil in a pan over medium heat. Sauté garlic until fragrant, then add onion and carrot. Cook until softened.

Add the mackerel fillets to the pan and lightly sear both sides. Pour in sake and let it evaporate slightly.

Add the canned tomatoes, water, bouillon granules, salt, and miso to the pan. Stir well to combine.

Simmer the mixture over low heat for 20 minutes, occasionally stirring. Add the cooked soybeans halfway through.