Grease a baking or casserole dish, and prepare the oven to heat up to 450F.
Put some oil in a saucepan and cook over medium heat. Next, add the onion, zucchini, and garlic, stir and cook for five minutes. Then, take the pot down and stir in the tomatoes, pepper, basil, and salt.
Place the Halibut steaks in the dish as one layer. Then, scoop equal amounts of the zucchini mixture on each steak, top with equal amounts of the cheese, and put the dish in the oven.
Bake the halibut steaks for 15 minutes. The fish should be ready when they flake easily with a fork. Serve hot.