Melt butter in casserole dish (I use about half stick of butter for a 2 quart casserole). Layer ingredients on order given, saving the soup and milk for last. Quantities depend on size of casserole you want to make. Continue to layer onions, chips, and cheese until you have the size casserole you want. Then pour soup diluted with milk over the top. I usually use enough milk to rinse out the soup can for a 2 quart casserole. If you're making a larger casserole, use more milk. Bake at 325 for 45 minutes.
Note: I use Zapp's Crawtator chips instead of BBQ flavor and I sprinkle cheese on top, since I love cheese. This casserole is always a big hit. Most people don't realize it's onions...a great dish!