I'd guess the mushrooms would be done with oyster sauce (did the gravy have a sweet/savoury taste?). Were the mushrooms the button variety?
Slice an onion, fry until soft (finely chopped ginger/garlic as you like), add mushrooms. In a small dish, mix couple of teaspoons of cornflour, dollop of oyster sauce and water - add to pan to make gravy (do a bit first, then add/adjust until you get the consistency/volume you want). Season to taste.