Preheat your oven to 375°F (190°C).
In a large skillet, heat the olive oil over medium heat. Add the diced onion and minced garlic, sautéing until the onion becomes translucent.
Add the ground beef to the skillet, cooking until it is browned and fully cooked. Drain any excess fat.
Stir in the marinara sauce, Italian seasoning, paprika, salt, and black pepper. Let the mixture simmer for a few minutes.
In a baking dish, layer half of the sliced zucchini at the bottom.
Spread half of the beef mixture over the zucchini, then sprinkle with half of the mozzarella and Parmesan cheese.
Repeat the layers with the remaining zucchini, beef mixture, and top with the remaining cheese.
If using, dollop the ricotta cheese on top.
Cover the dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and golden.
Garnish with fresh parsley before serving.
