This is an essential side for hot-dogs, burgers and even steak sandwiches. The trick is to saute the onions in their own juices, rather than frying them in oil. This is best made in quite large quantities - three or four onions - but don't worry because your guests will love it and will be back for more!
This is best cooked in a deep heavy saucepan with a non-stick lining and a well-fitting lid. Slice three or more onions into long narrow strips or rings - this makes it much easier to pick up a portion of onions with a pair of tongs.
Put a teaspoon of vegetable oil into the saucepan and add the onions, and cook over a very low heat for at least half an hour with the lid on. It should only be necessary to stir the onions every now and then, which should go soft and a very pale brown without becoming either burnt or sticky.
Optionally, you can add a clove of garlic - crushed or very finely chopped. Don't be tempted to add more garlic, since you want the dish to taste of onions, not garlic! You can also add a teaspoon of chopped dried mixed herbs, and/or a little finely-ground black pepper.
I prefer to cook this in the kitchen and bring it out to the BBQ to serve. The use of a heavy pan with a lid means that it will keep warm for ages. Of course, if you have a gas burner ring on your barbecue, then the pan can sit there over a low flame to keep warm.
Offer your guests onions on their hot dogs, sausages, burgers, or even BBQ steaks. Serve the onions with tongs.