You don't need onions or mushrooms, you can make gravy from strong stock, thickened using a roux or cornstarch slurry. This only really comes out well if you can get concentrated stock, it's not really that good with stock cubes, as they aren't really stock, more of a flavored salt.
My go-to gravy recipe uses Knorr stockpots, which are widely available in the UK and EU (from what I've seen in my travels), in the US they seem to be marketed as Homestyle stock, and they look like the same thing. I start with a roux made from 25g flour (about 2 tbsp) and 25g butter, cooked for about 2 minutes to give it a bit of color, then add 1 beef and 1 chicken stockpot and 600ml water, which is 2/3 of the normal dilution. I bring it up to a simmer, add some pepper, a drop of honey and a splash of dark soy for a bit of richness and color. If I have drippings I use those instead of butter to make the roux, if I have juices I will add this in addition to the base gravy recipe without adjusting it, it's just bonus flavor. If I have neither it still comes out well.
The flavor of the gravy comes from 2 things:
- The roux: cooking the flour down to give it some color gives depth of flavor. You can use a cornstart (cornflour) slurry after heating but I find you can tell the difference. You can cook the roux longer and get more flavoring, at some point you start to lose the thickening power. You can make more roux than you need and spoon it out, adding it until you get the thickness of gravy you like
- Concentration of the stock: you need a more powerful meaty flavor for gravy, thin stock won't cut it. If you have thin stock you can concentrate it down by gently simmering it to evaporate water.