Rachael Ray's Chicken Tetrazzini

1 small head cauliflower
8 ounces extra-wide egg noodles
1 stick (8 tablespoons) butter
8 ounces assorted fresh mushrooms or white mushrooms, sliced
4 cloves garlic, finely chopped
3 tablespoons all-purpose flour
2 cups chicken stock
1 cup heavy cream
3 to 4 cups shredded poached chicken breasts
1 cup panko
1 cup grated Parmigiano-Reggiano
1/4 cup chopped fresh flat-leaf parsley
1 large or 2 medium shallots, finely chopped
1/2 cup dry sherry (or dry white wine)

Preheat the oven to 400 degrees.
Bring 1 1/2 cups salted water to a boil and add cauliflower.
Cover and steam until just tender, about 7 to 8 minutes.
Strain and cut into florets.
Bring another pot of salted water to a boil and cook the noodles until almost al dente.
Strain and set aside.
Heat 4 tablespoons butter in a large skillet over medium heat.
Add the mushrooms and cook until tender and browned.
Add the garlic and shallots and cook, stirring for 2 to 3 minutes.
Add the flour, whisking for 1 minute.
Mix in the chicken stock and bring to a boil, scraping the bottom and sides for any browned bits.
Reduce the heat and allow to simmer for a few minutes, then stir in the cream.
Season with salt, pepper, nutmeg, and then add the cauliflower, noodles, and chicken.
Transfer to a casserole dish.
Melt the remaining 4 tablespoons of butter in a small bowl or pan and mix with the breadcrumbs.
Add the cheese to the breadcrumbs mixture and stir to combine.
Sprinkle the mixture over the casserole and bake for about 40 minutes or until hot and browned.
Sprinkle with parsley.