I make gravy from onions, it's sort of a pureed french onion soup thickened a bit. You can freeze the onion soup before it's gravy, and have gravy ready really fast. Here's a basic recipe, I rarely measure anything when making it, sorry.
Onion Soup base:
white onions, 4+ large ones you want to start with what seems like a LOT of onions
dark, Chinese style soy sauce or Bragg's aminos (for color)
Peel and thinly slice the onions.
Heat a tablespoon or two of olive oil in a tall stockpot. Add the onions and a teaspoon of salt, and cook on fairly high heat, stirring frequently. Cook the onion mass for at least a half hour, an hour is better. The onions should be browned on the edges. You don't want totally caramelized onions, you just want them sweated and a bit browned. Add a tablespoon or two of sherry and allow the alcohol to cook off. Add water to the level of the top of the mass of onions, a half teaspoon of soy sauce, some fresh thyme leaves stripped from the stems, and simmer for about 20 minutes. Taste and add salt as needed. If the color needs a boost, add tiny amounts of soy sauce until it looks good. You don't want a distinct soy flavor. Simmer to incorporate the final additions, serve hot as soup. Freezes very well. For gravy, allow to cool and puree in a blender or with an immersion blender.
To make gravy, make a dark roux with oil and AP flour and slowly stir cooled soup into the hot roux and allow to simmer and fully thicken.
This gravy also makes a good base for a vegetarian stew; cook chunks of potato, carrot, pearl onions, whole mushrooms, sliced sunchokes, etc. in it and add a few frozen peas just before serving.