Ingredients
2 1/2 cups milk
1/2 teaspoon dry mustard
1/2 teaspoon garlic salt
1 tablespoon butter
1/2 cup chopped onion
3 tablespoons melted butter
3 tablespoons all-purpose flour
3/4 pound Cheddar cheese, cubed
3 (10 ounce) packages frozen chopped spinach, thawed and drained
1 cup shredded Monterey Jack cheese

Directions
1 Preheat oven to 350 degrees F (175 degrees C).
2 In a medium saucepan over low heat, combine milk, dry mustard and garlic salt. Bring to a slow boil and simmer for 2 minutes.
3 In a small skillet or saucepan heat 1 tablespoon butter over medium heat. Saute onion until browned; stir into milk.
4 In a small skillet or saucepan combine 3 tablespoons melted butter with flour. Cook over medium-low heat until thickened; stir into milk.
5 To the milk mixture add the Cheddar cheese, mixing well to melt. Stir in spinach. Pour mixture into a 9x13 inch casserole dish and sprinkle with Monterey Jack cheese.
6 Bake in preheated oven for 10 minutes, or until heated through.