Heat the olive oil in a skillet over medium heat. Add the onion, mushrooms, garlic and zucchini. Cover, and cook stirring occasionally, until the onions are translucent, about 5 minutes.
Place halibut steaks into a shallow foil lined baking dish, bake tented for 10 minutes
Then top with the sauteed vegetables. Season with basil, salt and pepper. Cover the fillets with a layer of sliced tomato. Return to the oven untented, and bake for an additional 10 minutes, or until fish flakes easily with a fork.
Serve with roasted lemon asparagus, cauliflower mashed potatoes, garlic crostini.