While the chicken rests, pour the pan drippings into a shallow bowl or fat separator, leaving the onions in the pan. Using a fat separator or large spoon, skim off and discard as much of the fat as possible.

Pour the remaining drippings back into the roasting pan with the onions. Place pan over the medium-high heat, and cook for 1 minute. Add chicken stock, and raise the heat to high. Using a wooden spoon, deglaze the pan by stirring up any brown bits baked on to the bottom so that they combine with the stock; these brown bits add flavor to the gravy. Cook until the liquid is reduced by half, about 4 minutes.

Strain into a small bowl, pressing down on the onions with the back of a spoon to extract any remaining liquid; discard the onions. Stir in butter until it is fully incorporated. Serve immediately with the roasted chicken/turkey.