Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain.
Preheat oven to 350 degrees F (175 degrees C). Lightly grease a medium baking dish.
In a large bowl, mix turkey, pecans, parsley, egg, ricotta cheese, Parmesan cheese, salt, and white pepper. Stuff cooked shells with the mixture. Place shells in the prepared baking dish.
Melt butter in a medium saucepan over medium heat. Stir in shallots, and cook about 1 minute. Mix in flour, and cook about 2 minutes, stirring constantly. Gradually mix in chicken broth and sherry. Stir constantly until thickened. Stir in salt, white pepper, Gruyere cheese, and heavy whipping cream. Spoon over stuffed shells, and sprinkle with Parmesan cheese.
Bake in a preheated 350 degree F (175 degrees C) oven for 30 minutes, or until heated through and bubbly. Place under broiler to lightly brown. Serve immediately.