Peel and slice onions 1/4" thick, keeping the rings together. Transfer sliced onion with a pancake turner to a skillet. Saute onion in olive oil over medium heat, 2-3 minutes on each side, or until onions are translucent.
In a 13"x9"x2" roasting pan, arrange half of the onion slices to cover the bottom. Place fish on top of onions. Sprinkle olives, garlic, basil and pepper over the fish. Top with remaining onion slices and sliced tomatoes. Salt and pepper to taste.
Place pan with fish in center of oven, uncovered, and bake for 12-15 minutes, or until fish is opaque.