Coat a large skillet with fat-free cooking spray and heat over medium-high heat. Melt 3 teaspoons of the margarine in the skillet and stir in the onions, separating them into rings.
Cook and stir until softened, 2 to 3 minutes. Sprinkle with the salt and 1/4 teaspoon of the pepper. Reduce the heat to medium-low, cover, and cook, stirring frequently, until the onions are deep brown (approximately 15 minutes). Stir in the olives. Remove the mixture to a plate and cover with foil to keep warm.
Combine in a small bowl the red pepper and remaining 1/4 teaspoon black pepper. Cut the fish crosswise into 1-inch thick medallions.
Melt the remaining 1 teaspoon margarine over medium heat in the same skillet used to cook the onions. When hot, add the medallions and cook until opaque throughout, about 3 minutes each side.
Remove to a platter or plates and top with the onion mixture.
Sprinkle with the parsley and serve the lemon wedges for squeezing.