Lightly spray a large non-stick frying pan with oil and place over a medium to low heat. Add the sliced onions and cook for 25-30 minutes, stirring occasionally, until very soft, sweet and golden brown. If they start catching on the pan, add a small splash of water and stir to loosen any browned bits. Don't rush this stage, as the onions are what give the sauce its rich French onion flavour.