Melt butter in a saucepan and add the chopped onion and garlic. Cook the onion and garlic for about 5-7 minutes until tender.

Add flour and whisk until all the lumps have disappeared. Cook this mixture for about 4-5 minutes. Make sure to constantly stir so it does not scorch.

Add salt, pepper, and the milk to the roux (flour and butter) while whisking. Whisk until smooth.

Add the yellow cheddar. Cook until all cheese has melted. Add the cooked macaroni.

Place the mac and cheese in an oven-safe serving dish and top with the shredded white cheddar. Cook in the oven (325˚) until the cheese has melted and the edges have browned.