Hi Welshman,
My own base sauce uses only onions, garlic, ginger and tinned tomatoes and produces a very nice base gravy. Onions is key, if your base gravy is made of 90% onions, you're on the right track but you have to make sure that you give them a good long cook, if you want to release their sweetness that is.
Ray

p.s, I have used cauli before now, and it produced a very nice gravy.