Boil the potatoes in lightly salted water until tender. Cool and store in the fridge for up to one day.
Melt the butter in a saucepan.
Add the garlic and cook it for a minute over medium heat.
Whisk in the flour and cook for another minute.
Whisk in the hot milk, stirring constantly. Stir in the cheeses and keep stirring until it’s all melted and smooth.
Cool and store the finished cheese sauce in the refrigerator for up to a day.
Butter the inside of a slow cooker.
Slice potatoes into 1/4" slices and arrange in the crock.
Spoon cheese sauce over the top.
Cook on low for 4-5 hours.