Preheat the oven to 375°F and grease a 9x13 baking dish.
In a skillet over medium heat, cook the ground beef, onion, and garlic until browned. Drain excess grease.
In a large bowl, combine the cooked beef mixture, half the cheese, zucchini slices, marinara sauce, Italian seasoning, paprika, and red pepper flakes. Stir well.
Pour the mixture into the greased baking dish and spread evenly. Top with the remaining mozzarella and Parmesan.
Bake for 25-30 minutes, or until the cheese is bubbly and golden.
Drain Excess Grease: If using higher-fat ground beef, drain the excess grease after browning to prevent the casserole from becoming too oily.
Slice Zucchini Evenly: Cut the zucchini into even slices to ensure they cook uniformly. If they release too much moisture, pat them dry with a paper towel before mixing.
Use Freshly Shredded Cheese: Pre-shredded cheese contains anti-caking agents that can affect melting. Shredding fresh mozzarella and Parmesan will give the best texture.
Let it Rest Before Serving: Allow the casserole to sit for 5-10 minutes after baking to help it set and make serving easier.
Make It Spicier: Increase the red pepper flakes or add a dash of hot sauce to the meat mixture.