Heat oven to 400°F. Sprinkle both sides of each cod fillet with salt and pepper. In ungreased 13x9-inch (3-quart) glass baking dish, arrange fish in single layer.
Squeeze juice of 3 lemon quarters over fish. Arrange tomato slices on fish to cover, overlapping as necessary.
In 8-inch skillet, heat oil over medium heat. Add onion; cook 3 to 4 minutes, stirring frequently, until tender. Add garlic; cook and stir 1 to 2 minutes longer.
Remove from heat; stir in parsley. Sprinkle onion mixture evenly over tomatoes. Squeeze juice of 1 lemon quarter over onion mixture.
Bake uncovered 15 to 20 minutes or until fish flakes easily with fork. Serve fish with remaining lemon quarters.