Preheat oven to 350 degrees. Spray an 8-inch (or 7- by 11-inch) baking dish with nonstick cooking spray.
Cook pasta according to package directions; drain and set aside (to cool and dry out).
While the pasta is cooking, spray a large skillet with nonstick cooking spray and place over medium heat. Add the turkey sausage, carrot, zucchini, pepper and onion and cook for 7 to 8 minutes or until the sausage is no longer pink and the veggies are tender. Drain off fat.
Reduce heat to low and stir in cream cheese, basil, dill and garlic powder and keep stirring until combined and the cheese is melted. Season to taste with salt and pepper. Remove from heat.
Spread around 1/2 cup marinara sauce in the bottom of the prepared dish.
Spoon about 3 tablespoons of turkey mixture into 12 pasta shells and arrange, filled side up, over sauce in prepared dish. Spoon remaining 1 cup marinara sauce around and over the top of the stuffed shells. Bake, covered, for 30 to 40 minutes or until hot. Sprinkle with Parmesan cheese before serving.
Note: I suggest preparing 15 empty shells because it’s likely that one or two will tear in either the cooking or stuffing process. That way you will definitely have 12 stuffed shells in the end!
Make ahead: Shells can be prepared but not baked earlier in the day, covered and refrigerated. Bring to room temperature before cooking. Can also be baked, cooled, wrapped and frozen for up to 1 month.