Slice zuccini 1/4 inch thick, fresh motzerella and vidalia onion into rings. Layer in deep casserole dish adding salt and pepper with each layer
Put 1/4 c butter in sauce pan and let melt. Add 1/4 c flour to form rue. Add 3 c milk and whisk into rue stirring until mixture begins to thicken. Add Velveta cubed to white sauce and continue to whisk until cheese is melted. Pour over zuccini and onion. Place in fridge for 3 or 4 hours, OR you can pre-make it the night before, cover and leave over night. (Makes for a simpler Thanksgiving or Christmas)
Preheat oven to 350* with a cookie sheet inside. Place uncovered casserole onto cookie sheet. Bake for 1 hour or until top is nice and bubbly brown. Let sit for about 15 minutes before serving.