1½cupshredded cheesesharp cheddar, Fontina, Gruyere, Gouda, or a combination
8medium potatoespeeled and sliced thin with a mandoline or food processor
Make the cheese sauce: melt the butter in a medium saucepan. Stir the flour, salt and pepper. Gradually add the milk, whisking over medium heat until the sauce is thickened and bubbly. Add the garlic and 1 cup of the cheese, stirring to melt. Remove from the burner and set aside.
Butter the crock of a 4 to 6 qt. slow cooker. Layer half of the potatoes evenly on the bottom, and sprinkle all of the diced onions over the potatoes. Pour half of the cheese sauce over the layers, spreading as needed to cover all the potatoes.
Repeat with remaining potatoes and cheese sauce. Sprinkle the remaining 1/2 cup of cheese over the top.
Cover and cook on low 6 to 7 hours or on high for 4 hours.*
*Can be made up to a day in advance and refrigerated before cooking (after step 3). Start for 1 hour on high first if cooking on low to warm up from being refrigerated (add another 30 min to cook time if cooking on high).