Step 1

Position a rack in the center of the oven and heat the oven to 400°F

Step 2

Grease a 9x13-inch baking dish or a large cast iron skillet with butter. In 12 inch nonstick skillet over medium heat, melt the butter

Step 3

Add the leeks, stirring until leeks are completely coated

Step 4

Cover and cook, stirring occasionally, until the leeks are tender, about 20 minutes

Step 5

Add the milk, cream, blue cheese, garlic, mustard, nutmeg, salt and pepper and simmer uncovered

Step 6

Stir the mixture occasionally, until slightly thickened, about 15 minutes

Step 7

Be careful not to let boil

Step 8

Remove from heat, and allow to cool. In a large bowl mix the gruyere and parmesan cheese. Layer one third of the potatoes in the baking dish or skillet

Step 9

Pour one third of the leek mixture over the top

Step 10

Sprinkle one third of grated cheese and 1 tbs on chives

Step 11

Repeat to make two more layers reserving the last laying of chives. Cover Au Gratin with foil and bake for 45 minutes

Step 12

Remove cover and continue baking until potatoes are completely tender when poked with fork and the crust is golden brown, about 30 more minutes

Step 13

Allow to sit for 15 minutes after removing from oven

Step 14

Garnish with reserved chives and serve.