Prepare a grill for indirect heat. If using a charcoal grill, build the hot coals on one side only. If using a gas grill, heat one side only to medium-high heat.
Heat a cast-iron skillet over indirect heat. Melt the butter. Add the garlic with a pinch of salt and cook until it softens, about 3 minutes.
Meanwhile, grate the potatoes on the large holes of a box grater.
Add the evaporated milk, sour cream, thyme and hot sauce to the skillet and bring to a simmer. Add the potatoes and 1 cup of the cheese. Season with salt and pepper, move the skillet over indirect heat and cover the grill. Cook until the potatoes are just about cooked through, about 20 minutes.
Top the potatoes with the remaining 1 cup of cheese and close the lid. Continue to cook until the cheese is melted and the potatoes are completely cooked through, 8 to 10 minutes. Serve with the scallions sprinkled over the top.
(Alternatively, you can bake this in a 350 degrees F oven for 20 minutes, then top with the remaining cheese and bake for an additional 10 minutes.)
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