Slice 4-6 sweet potatoes and 4-6 russet potatoes into evenly sized slices (potato quantity depends on size)
In a large bowl, toss the potato slices with chopped garlic, leek, thyme, sage, salt and pepper.
Coat slow cooker with butter.
Layer potato/spice mixture in the bottom of the slow cooker, and sprinkle salt and pepper lightly between the potato layers.
For the cheese sauce, in a medium sauce pan, over medium-high heat, melt 4 Tbsp. of butter.
Slowly add 4 Tbsp. of flour and 1/2 tsp. of salt. Whisk for 1 minute.
Gradually stir in the whole milk.
Add in the shredded cheese, 1 cup at a time, melting the cheese between each cup.
Once the cheese is completely melted, pour over the potato layers.
Cook on low for 4-6 hours or until desired doneness is achieved.