Preheat the oven to 400 degrees F (200 degrees C).
Heat the olive oil in a skillet over medium heat. Add the onion, bell pepper, mushrooms, garlic and zucchini. Cover, and cook stirring occasionally, until the onions are translucent, about 5 minutes.
Place halibut steaks into a shallow baking dish, and top with the sauteed vegetables. Season with basil, salt and pepper.
Bake for 10 minutes in the preheated oven, then remove the dish, and cover the fillets with a layer of sliced tomato. Return to the oven, and bake for an additional 10 minutes, or until fish flakes easily with a fork.