6 tablespoons unsalted butter, divided
1 pound baby bella mushrooms, trimmed and sliced
1 small onion, diced
2 - 3 large cloves garlic, minced
1/2 cup dry white wine
3 cups cooked and shredded chicken (equivalent of 1 rotisserie chicken)
1/2 cup plus 2 tablespoons all-purpose flour
4 cups warmed milk, preferably whole milk
1 cup chicken stock
2 teaspoons kosher salt, plus more to taste
1/4 teaspoon dry mustard powder, plus more to taste
1 teaspoon dry sherry or marsala (optional)
Freshly ground black pepper
1 cup grated Parmesan, divided
16 lasagna noodles (from a 1 pound box)
2 cups shredded mozzarella cheese
2 cups grated gruyere cheese