The Basic Recipe
Put some hot grease in a pan.
Mix in some starch.
Gradually stir in some liquid.
Details
This is a cooking fundamental, the basic recipe for a whole bunch of sauces. For example, to make a white sauce, you stir flour into melted butter and then slowly add cream. The oil and starch get bound together, so that when you add water, both of them dissolve and make it thick and tasty.
Any kind of oil will work. I haven't tried starches other than wheat flour -- you'll have to experiment on your own. I mix in enough so that it's roughly as thick as gravy. The pan should be on low heat, so the flour and oil don't burn, but you're supposed to let it cook a minute to toast the flour a little. Then begin adding the liquid.
For some reason, when you add water to a hot oil-flour mixture, at first it makes it
thicker. You'll have to add quite a lot before it starts getting thin again. Also, unless your liquid is very hot, it's going to cool down the pan and you'll want to turn the heat up to medium or even high, depending on how fast you're adding it. The limit to how fast you can add it is how fast you can stir it in. Never add more liquid at once than the volume of stuff that's already in the pan, and stir it thoroughly before adding more. If you add liquid too fast or don't stir it enough, your gravy will be lumpy. When you get the thickness you want, you're done, unless you want to add more flavorings.
Meat Gravy
Your oil and water are right there in the roasting pan, but the proportions will probably not be right. If you've got lean meat with a lot of water-based drippings and not much fat, you'll need some extra oil. (I recommend cold pressed olive oil or butter.) More likely, your water-based drippings will have boiled down so far that you'll need some extra broth. You can make it ahead of time by stewing bones and scraps, or mix water with concentrated boullion, or buy broth in a carton. A trick for comparing the concentration of packaged broths is to look at the calories and protein on the nutrition panel.
When I bake a turkey, I take the bird out of the pan, then pour the drippings into a bowl or large glass, where it's easier (after it has separated again) to skim the oil. You're not going to separate the oil and water perfectly. A little water in the oil will not ruin the gravy, and oil left in the water will rise to the top of your eventual gravy where you can skim it if you want.
I put the oil in a clean pan large enough to hold all the gravy, then I add a few handfuls of flour and some black pepper, stir it for a minute, and begin stirring in the water-based drippings. Also I put water in the roasting pan to dissolve the stuff that's left in there, and stir that in, and if I still need more liquid, I'll use store-bought broth. At the end, I put in some unrefined salt.
Veggie Gravy
Vegetable broth that comes in cartons is seldom concentrated enough to make good gravy by itself, and it's not a good value. Neither are those "vegetable boullion" cubes. Every brand I've seen is mostly filler, typically "yeast extract" and salt and fat. What you want is powdered dehydrated vegetables, and you can get them cheaper in bulk at your local food co-op, and you can get yeast and salt and fat
much cheaper. Any powder of a vegetable you like will be great, if you can find it. I recommend onion powder because it has great gravy flavor and it's relatively cheap. Also, whatever spices you like, and lots of of nutritional yeast will help, but I've stopped eating it because it's high in purines which give me gout.
Now, to make the gravy, you heat up some fat of your choice. (Here's a page on the healthfulness of various
fats.) Then add flour or some experimental starch, and gradually add water. I've found that the best time to add the powdered veggies and yeast is about halfway through adding the water. Too soon or too late and they get lumpy. Also you can pre-dissolve them in hot water. Leafy spices and black pepper can be added at any time, but I like to put them in the oil at the beginning, so that their fat-soluble components get dissolved and distributed better.
Another option for veggie flavor is to is to make your own broth from boiling actual vegetables. This is an especially good idea if you have more vegetables than you could eat normally, for example if it's harvest time in your garden or you've just hit a good dumpster. I would pack as many veggies in the water as I could, boil the hell out of them, taste the various boiled remains to decide if I want to leave them in the gravy or compost them, then use what I have as the water in the general gravy recipe. If you're doing a lot of broth-making, get a pressure cooker. It's much faster and uses less energy.
Mushroom Gravy
Get a lot of mushrooms, slice them up, cook them in a
lot of oil, and when most of the water is cooked out, add your flour, and then water or broth or a carton of mushroom broth. Nutritional yeast is great here too.
(public domain, anti-copyright, last updated november 2013)