For filling: Pour 1 ½ tablespoons oil into a large skillet and place over medium-high heat. Add sausage and ground turkey and brown for 4 to 5 minutes, breaking up into small piece with a wooden spoon.
Strain meat from skillet and place into a large mixing bowl. Set aside.
Drain grease from skillet and place back over medium-high heat. Add remaining oil and sauté onion for 3 to 4 minute. Add garlic and herbs and continue to sauté for an additional 2 minutes.
Transfer mixture to bowl with the meat and allow mixture to cool, about 15 minutes.
Add remaining filling ingredients into the mixing bowl, season with salt and pepper and stir together until fully combined.
To assemble: Preheat oven to 375˚F. Pour and evenly spread 2 cups of marinara sauce into the baking dish and set aside.
Toss shells with remaining cup of marinara sauce until each is evenly coated.
Stuff each shell with 2 tablespoons of filling and place into the baking dish until all shells have been stuffed and baking dish is full.
Cover with foil and bake for 15 to 20 minutes. Uncover shells, sprinkle with shredded mozzarella and bake for an additional 10 minutes.
Remove shells from oven, cool for about 10 minutes and top with fresh basil leaves. Serve with extra grated Parmesan on the side.