Chop the chicken fat and chicken skin into approximately 1/2" pieces. (See the video for how to remove the skin from chicken thighs.)
Toss the chopped chicken fat and skin in a bowl with salt and pepper.
Place a frying pan onto a stovetop set to medium-low heat.
Add the chopped chicken fat and skin to the frying pan.
Cover the frying pan with a lid and allow the chicken fat and skin to begin to render.
After 15 minutes, remove the lid from the frying pan and increase the heat to medium.
If using, add the sliced onions at this time and stir well with the chopped chicken fat and skin.
Continue to allow the chicken fat and skin to render for 15-20 minutes while stirring occasionally. Watch the frying pan closely and do not allow the mixture to burn.
After approximately 15-20 minutes, the chicken fat should be dissolved, and the chicken skin should be a deep golden brown and very crispy.
If using, the onions should also be a deep golden brown color.
Turn off the stovetop burner and remove the frying pan to a heatproof surface.
Using a wire mesh strainer over a heatproof vessel, slowly pour the liquid from the frying pan into the strainer to catch any of the crispy chicken skin bits. (See video.)
Set the crispy chicken skin bits aside and decant the liquid into a jar with a tight-fitting lid. The rich golden color liquid in the jar is your schmaltz or rendered chicken fat.
Transfer the crispy chicken skin bits to a paper towel-lined plate and allow the bits to cool. Once cooled, transfer the bits to a container with a lid. The bits of fried chicken skin are known as gribenes. They are quite tasty and can be used in place of croutons to top a salad or other dish where you are looking for a delicious crunch.
Both the schmaltz and the crispy chicken bits must be stored in the refrigerator. The schmaltz will stay fresh for approximately 6 months. The crispy chicken bits should be consumed within one week.
Schmaltz can also be stored in the freezer and will stay fresh for up to one year.