Yes, you can use dehydrated French onion soup - in fact that's what I used to use before it disappeared from the UK supermarket shelves. Combine a packet of dried French onion soup mix with one cup of milk and half a cup of single or pouring cream and pour that over the casserole. However, milk has a tendency to boil up and overflow out of the dish. To protect your oven I would recommend that if you are going to do this you should place the casserole on a baking tray to catch any spills.