Arrange the garlic and onion skins on a baking sheet. Make sure you’re using only the outer, dried skins — not the next layer of onion that has some moisture still.
Bake the skins until very dark brown, about 30 minutes.
Transfer the skins into a food processor. Get your fingers damp and flick just a tiny bit of water into the food processor. (This will help keep the skins from just flying around inside.) Process until the skins are finely chopped.
Optional: for a finer mixture, sift through a sieve.