Onion salt is onion powder with salt added. It is very easy to substitute onion powder for onion salt by simply adding three parts salt to every one part onion powder. For example, to obtain 4 teaspoons of onion salt, combine 3 teaspoons salt with 1 teaspoon onion powder.
Onion powder is simply dried, ground onion. To add the flavor of an entire onion to a dish, 1 tablespoon of onion powder is the equivalent of one medium onion. It is also possible to add water or other liquids to onion powder to reconstitute it so that its texture is closer to the texture of fresh onion.