Crying is said to be good for the soul, but it's bad for your fingers when you're working with a sharp knife. That's a drawback of fresh onions, which are certainly tasty but can be inconvenient to work with. It's often worth keeping a jar of onion powder in your pantry for times when you're out of onions, or you just don't want to invest the time and tears right this minute. It's easy to use, and speeds your meal prep dramatically.
What It Is
All onion powder is, at the end of the day, is just onions that have been dehydrated and then ground to a fine texture. You'll see two types at the store, one that's a very fine powder and one that's granulated. They're basically the same thing, but the granulated kind isn't ground quite as finely. The powdery kind blends into spice mixes a bit more easily, but the larger particles of the granulated kind will hold their onion flavor for longer. Otherwise, they're identical. When you switch from onions to onion powder, what you're primarily giving up is a texture in your finished dish. Onion powder still adds the onion flavor you need, but it won't physically make up part of the meal the way fresh onions do.
Do the Conversion
You'll see varying recommendations for how much onion powder to use, but most fall into roughly the same range. For every half-cup of chopped onion your recipe calls for, or about one good-sized medium onion, you'll need a tablespoon of onion powder. There are three teaspoons in a tablespoon, so if your recipe calls for less than a whole onion, you'll just need to do the math. A full cup of onion would require 2 tablespoons of powder, which is 6 teaspoons, so – for example – if your recipe called for 1/3 cup of onion, you'd need 2 teaspoons. That's about the same as one small onion.
Use Onion Flakes Instead
If you don't have onion powder, but do find some onion flakes hiding in your pantry, you can use those as well. Flakes are bulkier, which means you don't get as much onion in a given spoonful, so you'll need to measure differently. You'll need about three times the volume of onion flakes to replace any given amount of onion powder, so for every teaspoon of powder, you'd use a tablespoon of flakes. If you don't want to do the additional math, the simpler answer is just to turn the flakes into powder with a spice grinder, or a mortar and pestle. You could even make a case for keeping the flakes on hand and powdering them as needed, so you don't need to make room for both in your spice cupboard.
Make Your Own
Those measurements make the assumption that your onion powder is reasonably fresh, which isn't necessarily the case. If you use the standard amount and you aren't getting the flavor you want, just keep adding onion powder until it tastes right. Alternatively, you can make your own so you always know it's fresh. You can dry them in your oven, if you wish, but it's simplest with a dehydrator. Cut the onions into thin slices and dry them at a low setting for three to nine hours, until they're perfectly dry and brittle. Then you can chop them into flakes or grind them into powder, and store them in an airtight jar or container for later use.