Created by Derf on October 13, 2012
Step 1: Preheat oven to 375f degrees. Whisk together in a bowl the milk, cheese, thyme and pepper; set aside.
Step 2: Melt butter in medium saucepan on medium heat; add flour; heat and stir for 1 minute. Slowly add the milk mixture; stirring constantlyl Heat and stir for about 7 minutes until boiling and thickened.
Step 3: Layer half of the potato, half of the yams, and half of the onion, in order given, in a greased 2 quart casserole; pour half of the milk mixture evenly over top. Repeat with remaining potato, yams,and onion; pour remaining milk mixture over.
Step 4: Bake at 375f degrees on the middle oven shelf for about 1 hour until potatoes are tender.