Preheat oven to 400 F.
In a heavy skillet, heat 3 tbsp extra virgin olive oil and sauté the onions over medium heat until the onions are translucent, 5-7 minutes.
Transfer the onions into a baking pan, forming 4 piles, about the size of the fish fillets.
Place the fish fillets over the onions and season with salt and pepper.
Sprinkle the garlic, herbs and olive slices over the fish .
Place tomato slices on top.
Drizzle with the remaining 1 tbsp extra virgin olive oil.
Place the pan in the center of the oven, and bake uncovered for 15 - 17 minutes, or until the fish is opaque.