Mince, onions, peppers - you name it, you turn your back for a moment and there it is burning to the bottom of the pan. Add a ladle of stock, water or wine. This cools it down immediately, and the liquid then boils so stops it burning. This is useful when sweating onions for risotto for example and they start to brown slightly - add a drop of white wine to stop them colouring and let it reduce before adding your risotto rice.
If it's a bad, thick encrusted burning however, if you do this you will incorporate the burnt flavour all through your food, and that's the last thing you want. So the answer is to cut your losses and swap pans - lift out what mince/ onions etc you can save and put it in a new pan and start again.
Another good tip, as I mention on the lamb and apricot post is not to thicken anything that is going to sit in the pan, or go into the oven for a couple of hours, like minced lamb for shepherds pie, or shin of beef for beef and ale pie. Thicken it after cooking and it doesn't burn on to your pan/ dish for 3 or 4 hours - much easier to clean. When the meat is cooked, drain the sauce through a colander into a saucepan, and put the meat to one side. Then thicken the sauce with beurre manie or cornflour, and drop the meat back in.
P.S. Kitchen tip #44 - how to make the best roast parsnips - is in draft form at the moment - keep a lookout.......