In a skillet, saute sliced zucchini with chopped onions in a bit of olive oil. Add crushed tomatoes and heat until the zucchini is cooked through. Meanwhile, cook the rotini to al dente. Drain and pour into a baking dish. Add the cooked vegetable mixture, and stir. Season with salt and pepper to taste, and sprinkle with grated Parmesan. Bake for about 10 minutes at 400 ° F until lightly browned. Serve immediately as a side dish to fish or meat.