Step 1

Preheat oven to 400 degrees F.  Butter a 2 quart casserole dish and set aside.

Step 2

Slice potatoes in 1/8" thin slices (I use this mandoline slicer), and set aside.

Step 3

Add butter to medium saucepan, and heat over MED heat.  When butter is melted, add garlic and saute for 1 minute, until fragrant.  Add flour, whisk, and cook 1 minute.  Slowly add milk, whisking continuously, until no lumps remain.  Whisk over MED heat until mixture is thick and coats the back of a spoon.

Step 4

Turn off heat, add cheeses, dry mustard, thyme, paprika, salt and pepper, and stir until smooth.

Step 5

Layer half the potatoes and onion slices in prepared casserole dish, pour approximately half of the cheese sauce over potatoes and let sit for a minute.  Repeat with remaining potatoes, onions, and cheese sauce.

Step 6

Sprinkle top of potatoes with gruyere, cheddar and Parmesan.  Spray aluminum foil with cooking spray, then cover the casserole dish (sprayed side down).  Place casserole on a baking sheet and bake for 60 minutes.

Step 7

Remove foil and bake uncovered an additional 25-30 minutes.  Potatoes should be tender and cheese melted.  Broil on HIGH for a minute or two for extra browning and crispiness.

Step 8

Sprinkle top with chives and parsley and serve.