Per 4 people:
And, of course, you need to collect the juices from whatever meat you're roasting. If you're frying, not roasting, then you'll want some red wine to deglaze pans with.
Caremelise the onions: Slice radially. Into a pan with hot water and some salt, lid on. High heat until the water is mostly gone, then low heat for 45 mins. You can do this the night before and store the results in the fridge.
You could add some worcestershire sauce, or mustard. Some even suggest adding demiglace. You don't need any of this stuff if you've got juices from the meat.
Some people suggest blending the contents of the trivet with stock; but this ends up more like a soup than a gravy.