Ah, gravy. The rich brown sauce that honestly most people could just drink straight, but manners require us to serve it on top of comfort foods such as mashed potatoes, roast poultry, stuffing, etc. The tricky part is what texture you like – if you prefer yours thinner or thicker, you’ll have to play with how much juices/stock you use, and the cooking time. Don’t worry – there’s really no such thing as a bad gravy.
Assuming you've roasted some meat or vegetables for this gravy, you'll definitely want to use the juices from the roasting pan, straight from the oven. If you don't have enough, add some stock or water to get the full amount - measure this out and set it aside before starting so it's ready to be added! (If you don't have any roasting juices, obviously, use stock instead.)
In a small-to-medium sauce pot, melt the butter over medium-high heat while stirring. Once melted, sprinkle in the flour one spoonful at a time while constantly stirring or whisking - you want to combine the flour as you go but don't go too slow or you'll start to burn the bottom of the pot.
Sprinkle in the onion powder and keep stirring - the roux (you're making roux, yes) will start looking like cookie dough, light-colored, but will brown as you continue to cook it. It is your decision how brown you want your roux to be (and how "dark" the flavor) before you...
ADD THEM JUICES! Stirring/whisking constantly, evenly pour in your roasting juices/stock.
Stir in that Gravy Seasoning! Cook for 6 minutes, stirring/whisking constantly to "get out the lumps". Taste test it now and add salt until you get the saltiness you like. If the gravy is too thin, sprinkle in more all-purpose flour and whisk thoroughly - but remember the sauce will get thicker when it cools!
At this point, it's worth mentioning that immersion blenders are really fantastic for making gravy - here's where you'd want to thoroughly blend everything and get the 'lumpiness' of your gravy down to where you like it.
Fill up your gravy boat or ladle that awesomesauce over your food. Either way, go crazy.