gravy is more of a technique thing because there are so many variations.
I use a roux and thin it out. for roux you need a fat and flour.
the obvious candidates for 'the fat' -
- pork renderings
- chicken renderings
- butter
for pork and chicken you want to deglaze the pan to get 'the fond' flavors.
last couple years I've found it tricky to do from scratch using pork sausage - loose or cut out of the casing. it appears the fat content has been reduced in the name of "sausage is good for you." save your bacon grease/drippings - excellent flavor source if the sausage itself does not produce enough rendered fat.
chicken fat from roasting/broiling is easier to "acquire"
mix by volume - and by eyeball - it jis' ain't so critical... - equal parts flour and fat. for two people, 3 T each will work generously. any recipe that calls for flour in cups, you'll need to invite over your local friendly Army Corp . . .
it is important to cook the fat and the flour together for roughly ten minutes. otherwise you get a raw pasty flour taste. let it bubble a bit, it can go slightly brown, but you don't want it to go dark.
after that you whisk in / add milk - cream for the holidays.... - to get to the desired consistency. add in steps - don't just dump a whole bunch of liquid in. very easy to add more, very very tricky to wring it out....
taste for seasoning. depending on the sausage / chicken / etc you'll have some "base" of flavors. fresh ground black pepper and salt are the usual additional needs. to kick it up, cayenne pepper. past that all kinds of things are possible:
celery seed
caraway seed
finely minced garlic
minced green/red pepper
minced shallots/onion/scallion - scallion greens
chopped parsley
the list is near endless.
I never tell anybody this,,, but for a dinner type meal, I've sneaked in a dollop of horseradish to rave reviews. horseradish and anchovies / paste - two things you never want to tell anybody about. people who have never eaten it hate it - even tho they just chowed down on the mmmphmmph-this-is-really-good-stuff
oh, I also keep a roll of light and dark roux frozen/wrapped in foil - just in case the volume is insufficient. I can hack off a tsp or two to increase the volume 'on the fly'