Strain pan drippings and add enough water or stock to make 2 cups. Place in a pot and reduce the liquid to thicken a bit by boiling down for about ten minutes.
Add onion to liquid and finely puree with a stick blender or high speed blender. Cook the onion for a few minutes to further reduce the liquid.
Remove from heat and slowly pour about 1/4 cup of the hot liquid into the egg yolks while whisking to temper them. Gently pour the egg yolk mixture into the hot liquid in the pot while whisking.
Continue to whisk and simmer the gravy liquid. Whisk in the coconut oil or butter and stir until smooth. You can continue to simmer. Butter tends to thicken the gravy better than coconut oil, but the gravy will thicken more as it cools with the coconut oil.
If the gravy is not as thick as you'd like, you can add in a little glucomannan, gelatin, coconut flour, or xanthan gum to get the right consistency.For a lower carb gravy, the onion can be left out, but another thickener like gelatin will likely need to be used or the liquid may be too thin.