Instead of turkey, you can use rotisserie chicken in this comforting casserole. If you prefer, you can substitute chopped celery and carrots for the mushrooms. Slideshow: More Leftover Recipes Recipes
Preheat the oven to 375°. In a large pot of boiling salted water, cook the egg noodles about 2 minutes short of al dente. Drain the noodles well, give them a quick rinse under cold running water and drain again.
Generously butter a deep 13-by-9-inch baking dish. In a medium microwave-safe bowl, melt 3 tablespoons of the butter. Add the panko, cheese and 1/4 teaspoon each of salt and pepper and stir to combine. Dust the greased dish with 3 tablespoons of the crumbs. Set the remaining crumbs aside.
In a large, deep skillet, melt the remaining 4 tablespoons of butter over moderate heat. Add the onions and cook, stirring occasionally, until the onions just begin to soften, about 3 minutes. Add the mushrooms and cook until the liquid the mushrooms give off has almost evaporated, about 5 minutes. Add the sherry, increase the heat to moderately high and boil until the liquid is reduced by half, about 2 minutes. Add the flour and cook over moderate heat, stirring constantly, until moistened, about 1 minute. Add the broth, bring to a simmer and cook, stirring, until thickened, about 3 minutes. Stir in the pasta, turkey, herbs and lemon zest and season with salt and pepper.
Spread the contents of the skillet into the prepared baking dish. Sprinkle the reserved breadcrumbs evenly over the top. Bake the casserole for about 30 minutes, until golden brown on top. Serve hot.
Turkey Tetrazzini can be assembled and refrigerated, covered, for up to 1 day.