Cook macaroni until almost done according to package instructions.

Preheat oven to 350°F (175°C).

Melt butter in a saucepan over medium heat.

Whisk in flour and cook for 2-3 minutes to form a roux.

Gradually whisk in milk, stirring constantly to prevent lumps.

Continue cooking over medium heat for about 10 minutes, stirring occasionally, until the sauce thickens slightly.

The sauce should be slightly thicker than milk.

Remove from heat and stir in salt, pepper, shredded cheddar cheese, and shredded Monterey Jack cheese until melted and smooth.

Add the cooked macaroni to the cheese sauce and stir to combine.

Pour the macaroni and cheese mixture into a greased baking dish.

Bake in the preheated oven for 20 minutes, or until bubbly and golden brown.

Optionally, broil for about 3 minutes to brown the top further, watching carefully to prevent burning.

Let sit for about 5 minutes before serving.