Cut the onion in half. Slice one half thinly and grate the other half. Cut the cabbage into bite-sized pieces. Halve the bell peppers, remove stems and seeds, and slice thinly. Julienne the ginger.

Pat the salmon fillets dry with a paper towel and cut them into bite-sized pieces.

Layer the cabbage, sliced onion, and bell peppers in a frying pan. Place the salmon on top, sprinkle with ginger, salt, and pepper, and drizzle sake. Cover and steam over low heat for about 6 minutes until the salmon is cooked through. Transfer to a serving plate.

Wipe the frying pan clean with a paper towel. Add the grated onion and sauce ingredients (sake, mirin, soy sauce, garlic). Cook over medium heat, stirring, until the sauce thickens.