Step 1

Set the oven rack to the middle position and preheat to 400ºF.

Step 2

Melt butter in a large skillet over medium-low heat. Add garlic and onions, saute until fragrant, 2 minutes.

Step 3

Add flour, whisk to combine, stir and cook for 2 minutes.

Step 4

Gradually whisk in milk and cook until thickened over medium heat, 2 to 3 minutes.

Step 5

Turn off heat and whisk in salt, pepper, nutmeg, ½ cup gruyere, and ½ cup sharp cheddar cheese.

Step 6

Grease the bottom and sides of an 8-inch square casserole dish.

Step 7

Layer half of the potatoes, in overlapping rows in the dish. Spread half of the cheese sauce over the potatoes. Sprinkle ¼ cup gruyere and ¼ cup cheddar over the sauce.

Step 8

Layer the remaining potatoes, followed by cheese sauce, and sprinkle with the remaining gruyere and cheddar cheese.

Step 9

Cover with foil and place on a baking sheet. Bake for 30 minutes, remove the foil and bake until potatoes are tender and cheese is browned and bubbly, 20 to 25 minutes.

Step 10

Garnish potatoes with chives.