Combine soup, butter, sour cream or yogurt, milk, cheese, onion, salt and pepper in a large mixing bowl. Mix well.
Add the diced potatoes to bowl and stir gently until combined.
Place potato mixture in a 2-3 quart casserole dish or 9 x 13-inch baking pan.
Optional: Mix 2 T melted butter with crushed corn flakes or panko bread crumbs. If using panko, season with a little salt and pepper. Sprinkle crumb mixture over potato mixture. Also optional: sprinkle more cheese over the top.
Bake uncovered at 350 degrees for 45-60 minutes or until hot and bubbly.