Use 4-quart slow cooker. Add melted butter to the insert and swirl the sliced onions around in it. Pour in the beef stock. Cover and cook on LOW for 6 hours, or until the onion is translucent. Float the pieces of toast in the pot, and place the slices of cheese on top of the toast. Cover and let the bread sit in the slow cooker for about 10 minutes, or until the cheese has melted in the heat.