Step 1

Preheat oven to 350 degrees. Grease a 13 by 9-inch baking dish.

Step 2

Melt butter in a medium saucepan. Add the onion and saute 3 minutes.

Step 3

Add garlic, saute 30 seconds.

Step 4

Add flour and saute another minute.

Step 5

Slowly add milk to the flour mixture, whisking continuously. Bring to a gentle boil over medium-high heat, stirring frequently.

Step 6

Remove pan from heat and stir in both cheeses.

Step 7

Season with salt and pepper. Cover the pan and set aside.

Step 8

Wash potatoes and slice (do not peeinto 1/6-inch slices using a mandolin. Spread half of the potatoes into an even layer in prepared baking dish, making sure slices are separated.

Step 9

Pour half the sauce evenly over potatoes.

Step 10

Spread remaining potatoes over the sauce. Pour remaining sauce over the top of the potatoes.

Step 11

Bake in preheated oven until potatoes are tender when pierced with a fork, about 55 - 70 minutes. For browner potatoes, pop under the broiler for 2-3 minutes, watching them carefully.

Step 12

Sprinkle with chopped parsley.