Step 1

Preheat oven to 375 F. Lightly grease a 9 x 9 inch baking dish. Layer potatoes in the baking dish.

Step 2

Melt butter in a large saucepan over medium heat. Add onion and garlic and cook until soft and translucent, about 3 minutes. Whisk in flour and continue to whisk until browned, about 30 seconds to 1 minute. Gradually whisk in milk, and add salt and pepper. Continuously whisk until mixture has thickened, about 2-3 minutes. Turn heat down to low and add cheeses and stir continuously until melted, about 30 seconds.

Step 3

Pour cheese sauce on top of the potatoes, and cover with aluminum foil. Place into oven and bake for 1 hour. Uncover and continue to bake for 20-30 minutes or until lightly browned and the potatoes are tender and mixture has thickened.

Step 4

Garnish with fresh ground black pepper and green onions (optional) and serve immediately.