Soy and Ginger Steamed Fish

Name: Soy and Ginger Steamed Fish
Ingredients: 2 (6–8-oz.) skinless black bass fillets; 1 (4x3”) piece dried kombu (optional); 2 Tbsp. sake or dry white wine; 2 Tbsp. soy sauce; 1 Tbsp. mirin; ½ medium head of Napa cabbage, stems thinly sliced, leaves torn; 4 oz. mixed mushrooms (such as shiitake, oyster, beech, and/or maitake), torn into pieces; 1 (2”) piece ginger, peeled, cut into thin matchsticks; Cooked rice (for serving); 1 Tbsp. toasted sesame oil; 2 scallions, thinly sliced on a diagonal