In a medium saucepan over medium heat, melt the butter. Whisk in the flour and cook for 1 minute. Slowly whisk in the milk and stir until the sauce is thick and creamy. If the sauce is too thick, add more milk to thin it out. (You are looking for an egg nog consistency).
Reduce the heat to low and stir in the salt, cumin and cheeses. Stir until the cheese is melted. Remove from heat.