Preheat the oven to 375°F.
Cook whole wheat pasta shells according to package instructions, drain and set aside.
In a cast iron skillet or other oven safe skillet, heat the oil over medium heat, add the onions and ground turkey and cook until the turkey is browned, about 5 minutes. Stir in the garlic and Italian seasoning, and cook for another 2 minutes until well combined.
Add in the marinara sauce, fresh basil and then stir in the cooked pasta.
Top with mozzarella cheese and bake in the preheated oven until the cheese is melted and bubbly, about 15-20 minutes.
Garnish with fresh basil and serve immediately.