Soy and Ginger Steamed Fish

Name: Soy and Ginger Steamed Fish
Ingredients: 2 6–8-oz. skinless black bass fillets; Kosher salt; 1 4x3 piece dried kombu (optional); 2 Tbsp. sake; 2 Tbsp. soy sauce; 1 Tbsp. mirin (sweet Japanese rice wine); ¼ large or ½ medium head of Napa cabbage, stems thinly sliced crosswise, leaves torn if large (about 5 cups); 4 oz. mixed mushrooms (such as shiitake, oyster, beech, and/or maitake), torn into pieces; 1 2 piece ginger, peeled, cut into thin matchsticks; 1 Tbsp. toasted sesame oil; 2 scallions, thinly sliced; Cooked rice (for serving)