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Preheat oven to 400 degrees F. Butter a 2 quart casserole dish and set aside.
Slice potatoes in 1/8" thin slices (I use this mandoline slicer), and set aside.
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.
Turn off heat, add cheeses, dry mustard, thyme, paprika, salt and pepper, and stir until smooth.
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.
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.
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.
Sprinkle top with chives and parsley and serve.