Switch on the oven with the “grill” function to preheat to the maximum temperature, set the wire shelf to the highest position.
Place the fish fillets in a large non-metallic bowl. In another bowl, combine butter, minced garlic, red pepper flakes, half a parsley, lemon juice, and black pepper. Pour this marinade over the fish, mix well with your hands.
Heat butter in a large skillet over medium-high heat. Reduce heat to low. Cook oil until golden brown, about 2 minutes. Put the sliced garlic in the butter, cook for about 2 minutes, until golden brown, but not brown. Add capers and bell peppers to the pan and stir.
Sprinkle the fish on all sides with salt and pepper, put on a baking sheet, place under the grill, cook for about 5 minutes on each side. Transfer the fish to a serving plate.
Heat the sauce over medium heat. Add balsamic vinegar, leftover parsley, and black pepper to taste. Place the sauce over the fish and serve immediately.