Melt 4 tablespoons butter and saute 2 tablespoons chopped onion. Add 2 tablespoons flour and stir; add 3/4 cup condensed milk and cook until thickened. Add 6 ounces smoked cheddar cheese (grated), 1 teaspoon each celery salt, garlic salt and Worcestershire sauce. Stir in 2 package thawed, chopped spinach. Pour into 2 quart casserole and top with seasoned bread crumbs. Bake at 350 degrees for 45 minutes. Prepare the day before. May also be frozen. Heat and serve.