Preheat the oven to 375°F (190°C).
Cook the penne pasta according to package instructions until al dente. Drain and set aside.
In a large skillet, heat olive oil over medium heat.
Add diced onion and cook for 3-4 minutes until softened.
Add minced garlic and cook for 30 seconds until fragrant.
Add ground turkey to the skillet. Cook, breaking it apart with a spoon, until browned and cooked through, about 6-8 minutes.
Stir in marinara sauce, dried Italian seasoning, salt, and black pepper. Simmer for 5 minutes to combine flavors.
In a large mixing bowl, combine the cooked pasta and the turkey sauce mixture. Stir well to coat the pasta evenly.
Transfer half of the pasta mixture into a 9×13-inch baking dish.
Sprinkle half of the shredded mozzarella and half of the Parmesan cheese over the pasta layer.
Add the remaining pasta mixture on top, then sprinkle the remaining mozzarella and Parmesan cheeses evenly over the casserole.
Bake uncovered for 20 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let it rest for 5 minutes.
Garnish with chopped fresh parsley before serving.