In a large skillet, crumble and brown the sausage, then drain off the grease.
Cut potatoes into fat 1/4" slices and boil for 10-15 minutes.
Place the sliced potatoes in a large saucepan and cover them with cold water.
Bring the water to a boil, then reduce heat and cook for about 8. Take care not to over cook the potatoes. They should be a little hard as they will continue to cook when you bake the casserole.
In a mixing bowl, combine the cream of potato soup with the milk, whisk together. Season with salt and pepper to taste.
Butter a 9×13 casserole dish. (Alternatively, you can use (2) 8x8 or 9x9 pans).
Begin layering the ingredients in this order: sauce on bottom, potatoes, sausage, cheese, and repeat until dish is full.
Pour the remaining sauce mixture over the top.
Crush the crackers and sprinkle them over the top.
Drizzle melted butter over the top, then finish off with a bit more cheese.
Cover with tinfoil and bake at 350-degrees F for 45-60 minutes until bubbly.
Remove the tinfoil and bake another 5-8 minutes to allow top to brown.