Step 1: Preheat your oven to 375°F (190°C).
Step 2: In a large mixing bowl, combine the ground turkey or chicken, Parmesan cheese, eggs, grated onion, minced garlic, chopped parsley, Italian seasoning, ground black pepper, and crushed red pepper flakes. Season with kosher salt to taste and mix well.
Step 3: Shape the mixture into meatballs, about 1-2 inches in diameter, and place them on a baking sheet lined with parchment paper.
Step 4: Bake the meatballs for about 20 minutes, or until cooked through and golden brown.
Step 5: Spread marinara sauce evenly in the bottom of your baking dish.
Step 6: Place the baked meatballs on top of the marinara sauce.
Step 7: Dollop ricotta cheese over the meatballs.
Step 8: Sprinkle shredded mozzarella cheese over the ricotta layer.
Step 9: Bake the casserole for an additional 20-25 minutes, or until the cheese is bubbly and golden brown.
Step 10: Let the casserole cool for a few minutes and garnish with chopped basil before serving.