Preheat the oven to 450 degrees . Grease the bottom of a 9-by-13-inch baking dish with olive oil. Toss together the cabbage, onion, scallions and cider vinegar in the baking dish; season with salt and pepper.
Brush the fish fillets on both sides with the remaining 2 tablespoons olive oil and season lightly with salt. Place the fish, skinned side down, on top of the cabbage mixture. Top the fish with the salsa to cover. Bake until the fish is opaque in the thickest part, 25 to 30 minutes.