Light a gas burner or flame grill. Hold a pepper with tongs directly over the flame and roast until blackened, rotating for even blistering. Place pepper in a paper bag and loosely fold bag. Repeat with remaining peppers. When cool, peel and seed. Cut 4 peppers into 1-inch squares. Reserve scraps. (Or, if you have only an electric oven, roast peppers in a baking pan for 45 minutes at 350°F, rotating them every 15 minutes.)

Chop remaining 2 peeled peppers and place in blender along with reserved scraps. Add ¼ cup of the oil and 2 tablespoons of the vinegar. Puree until smooth. Season with salt and pepper.

Preheat oven to 325°F.

In a shallow pan over low heat, combine butter, onion rings, ¼ cup water, and a sprinkle each of salt and pepper. Cover pot and cook until onions are tender, about 20 minutes. Drain. Add pepper purée and stir to bind onions with purée.

In a bowl, whisk together miso, remaining 2 tablespoons vinegar, and 2 tablespoons oil, mustard, 1/8 teaspoon salt, and several twists of ground pepper. Arrange fillets on a lightly oiled baking sheet. Brush a generous amount of miso dressing on the top sides of the fillets. Bake 8 to 10 minutes for medium-rare. Transfer fillets to a broiler and broil until tops are lightly browned, about 4 minutes.

To serve, dollop some onion-pepper mixture on each of 4 plates. Lay a salmon fillet on the sauce and garnish servings with roasted cubanelle pepper squares. Serve immediately.