1. Preheat oven to 375ºF. Butter a 3 quart baking dish. Set aside.
2. Bring a large pot of salted water to boil. Cook the pasta according to package directions for al dente. Drain. Add to a large mixing bowl. Set aside.
3. Meanwhile, in a large sauté pan over medium-high heat, melt 3 tablespoons of butter. Stir in the mushrooms and Herbes de Provence seasoning. Cook 5-7 minutes, stirring occasionally until the mushrooms are golden brown. Add the mushrooms to the bowl with the pasta.
4. Using the same sauté pan, melt 4 tablespoons of butter. Whisk in the four and cook until the mixture is toasted, about 2 minutes. Slowly whisk in the milk, cream, stock, sherry and Nutmeg. Bring to a simmer over medium heat, whisking constantly for 6-8 minutes.
5. Add the sauce to the mixing bowl with the pasta and mushrooms. Stir in the turkey, peas, parmesan, and lemon juice. Season to taste with salt and pepper. Transfer to the prepared pan.
6. In a small bowl combine the gruyere cheese and panko. Sprinkle over casserole. Dot with the remaining tablespoon of butter.
7. Bake for 30 to 40 minutes or until top of golden brown. Cool 10 minutes before serving.