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 every couple layers.
For the cheese sauce, in a medium sauce pan, melt 4 Tbsp. butter over medium-high heat.
Slowly add 4 Tbsp. of flour and ½ tsp. 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.
Place the lid on the slow cooker and cook for 4-6 hours on low, or until potatoes are soft.