Swedish Meatball Pasta Bake

4 tablespoons butter
2/3 cup yellow onion, finely chopped
2 teaspoons salt
1/2 teaspoon pepper
1/2 teaspoon ground allspice
1/4 teaspoon nutmeg
3 cloves of garlic, minced
2/3 cup milk
2 eggs
1 pound ground beef (I used 85/15.)
1 pound ground pork
3 tablespoons oil
Ingredients for the Pasta:

2 cups shell pasta
Ingredients for the Sauce:

6 tablespoons butter
1/4 cup plus 3 tablespoons all-purpose flour
4 cups beef broth (I used low sodium, equivalent to a 32 ounce box.)
1/2 cup water
1 tablespoon Worcestershire sauce * (See Note below.)