Trim the spinach roots and cut into 3cm pieces. Thinly slice the garlic.

Season the yellowtail fillets with salt and pepper, then coat lightly with flour.

Heat olive oil and garlic in a frying pan over medium heat. Once the garlic turns golden, remove it. Add the yellowtail and onion slices, cooking until browned. Flip, cover, and steam on low heat for 3 minutes. Transfer to a plate.

In the same pan, add the sauce ingredients (sake, mirin, soy sauce) and bring to a boil. Cook until the sauce thickens slightly. Transfer to a separate container.

In the same pan, melt the butter over high heat. Add spinach and frozen corn, stir-frying quickly until the spinach wilts. Season with salt and pepper.