I find gravy to be one of the easiest sauces to do quickly and with whatever I have on hand (1:1:8 ratio for fat, flour and liquid).
I made a brown sauce for poutine and some other potato dishes kind of like this the other day: melt butter at medium low heat in a small pot, say about 2 tbsp, add equal amount of flour by volume and fry until getting a little nutty. Whisk in broth or stock (I just use Better Than Bouillon) until consistency of thin gravy achieved (about a cup) and it begins to bubble. Add light and dark soy sauces for flavour and colour to taste. It’ll thicken as it cools.