Spinach Marie

Preheat oven to 350 degrees F (175 degrees C).
In a medium saucepan over low heat, combine milk, dry mustard and garlic salt. Bring to a slow boil and simmer for 2 minutes.
In a small skillet or saucepan heat 1 tablespoon butter over medium heat. Saute onion until browned; stir into milk.
In a small skillet or saucepan combine 3 tablespoons melted butter with flour. Cook over medium-low heat until thickened; stir into milk.
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.
Bake in preheated oven for 10 minutes, or until heated through.