(PREPARING THE CHEESE SAUCE) Turn on your stove to a medium temperature. In a medium-large saucepan, add the butter and let it melt. Once melted, add the milk and the half and half. Stir these until they sort of mix with the butter.(There may be slight discoloration.) Add the mexican cheese and give it a quick stir. Add the cheddar cheese. Mix everything together and bring your sauce down to a simmer. Let simmer for about 8 min, stirring occasionally. Preheat your oven to 350 F.