Preheat. Preheat your smoker to 275 degrees F.
Make the cheesy sauce. Prepare your sauce. In a large saucepan, melt butter over medium heat. Add in diced onion and minced garlic, then stir while cooking for 2-3 minutes, or until onions are translucent and fragrant. Whisk the flour into the sauce until no flour lumps remain. Continue whisking while adding in the chicken stock, milk, heavy cream, and sour cream. Bring to a boil for 1-2 minutes. Turn off the heat and stir in the salt, pepper, and Parmesan cheese.
Assemble the scalloped potatoes. Spray a 12 inch cast iron skillet with cooking spray. Layer ⅓ of your sliced potatoes in the bottom of the pan and season with salt and pepper. Spoon ⅓ of your sauce mixture over your potatoes. Add a second layer of overlapping potato slices, season with salt and pepper, and top with another ⅓ of the sauce mixture. Create a final layer of potatoes, salt and pepper, and the remaining sauce.
Smoke. Place the pan of potatoes on the smoker, close the lid, and cook for 1 hour. Cover the pan tightly with foil and bake for an additional 90 minutes, or until potatoes are tender.
Broil (Optional). For a crispy, bubbly top, remove the foil and broil the top of the potatoes in your oven for 2-3 minutes.