2 cups cubed fully cooked ham
4 cups shredded cooked chicken
1 cup shredded Swiss cheese
1 large onion, chopped
1/3 cup butter, cubed
1/3 cup all-purpose flour
1/8 teaspoon ground mustard
1/8 teaspoon ground nutmeg
1-3/4 cups whole milk
TOPPING:
1-1/2 cups soft bread crumbs
1/2 cup shredded Swiss cheese
1/4 cup butter, melted