Combine grated cheeses in a bowl, set aside. Saute chopped onions in ¼ c. melted butter over medium heat until opaque.
In large bowl, whisk together eggs, ¼ c. melted butter, both milks, salt, dry mustard, and WannaRub. Stir in pasta, cooked onions, and 3 cups of cheese mixture. Mix well and pour mixture into a lightly greased 3-quart slow cooker; sprinkle with ¾ cup cheese mixture, saving ¼ cup when cooking time is finished. Cover and cook on low for 3 – 4 hours but check after 2-1/2 hours. If more time is needed, check every 20 minutes for doneness. If using large oval crockpot, cooking time will be approximately 2 hours.