Preheat the Oven: Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish with cooking spray or a small amount of oil.
Cook the Ground Beef: In a large skillet, brown the ground beef over medium heat. Add chopped onion and garlic to the pan, cooking until the beef is fully cooked and the onions are softened. Drain any excess fat from the beef.
Add Seasonings: Stir in the tomato sauce, oregano, basil, salt, and pepper. Let the mixture simmer for 10 minutes to allow the flavors to meld together.
Prepare the Spinach: While the beef is simmering, sauté the spinach in a separate pan over medium heat until wilted (or, if using frozen spinach, make sure it’s thoroughly thawed and drained).
Assemble the Casserole: Layer the beef mixture and sautéed spinach in the greased baking dish. Top with shredded mozzarella and cheddar cheese. Optionally, sprinkle breadcrumbs over the top for extra crunch.
Bake the Casserole: Bake the casserole in the preheated oven for 20-25 minutes, or until the cheese is melted, bubbly, and golden brown.
Serve: Remove the casserole from the oven and let it cool for a few minutes before serving. Enjoy your delicious Spinach Beef Casserole!