Heat oil in skillet on medium heat. Add onions and cook 2-3 minutes until softened but not brown.
1-2 tablespoons olive oil, 1 onion
Add ground beef. Break up the meat and cook, stirring regularly, until fully brown, about 4-5 minutes. (Drain any excess fat at this point, if applicable)
1 pound ground beef
Add Italian seasoning, salt and pepper and minced garlic. Stir and cook for one minute.
1 tablespoon Italian seasoning, 1 teaspoon salt, ½ teaspoon black pepper, 3 garlic cloves
Add diced tomatoes, tomato paste and water. Mix well to make a sauce. Bring to a boil, then lower the heat and simmer, uncovered, for 7-10 minutes.
14 oz can diced tomatoes, 2 tablespoons tomato paste, ½ cup water
Add Parmesan cheese and zucchini and stir until well coated in sauce. Remove from heat.
½ cup Parmesan cheese, 1 pound zucchini
Transfer mixture to a 2- to 3- quart casserole dish. Top with shredded Italian cheese.
1 cup cheese
Cover and bake in a 375F oven for 30 minutes. Remove cover and bake an additional 10 to 15 minutes uncovered. Remove from oven and allow to rest at least 10 minutes before serving.