Chop up two or three onions, depending on how gravy-greedy you are, and fry them in a saucepan with a little oil. It doesn't matter what your knife skills are like because these bits of onion are going to get blasted by your stick mixer or food process/blender anyway so it's really no biggie unless you're trying to impress someone while you're cooking ;-)
Once the onions are about two thirds cooked (bearing in mind you're going to cook them until they are almost caramelised, add a handful of diced mushrooms (optional, it is fine with just the onions), any kind will do, even the dried ones that you soak to re constitute add great flavour - make sure you keep the soaking water to add later too!