Melt butter in a medium saucepan over medium heat, then add onions. Cook, stirring occasionally, until onions are translucent, about 5 minutes. Sprinkle the flour on the onions, stirring constantly, and cook until flour is golden to brown. Adjust heat so mixture does not burn.
Gradually whisk in 4 cups stock until mixture thickens and is smooth. If it is too thick, add liquid. Cool, cover and chill.
When ready to serve, reheat mixture over low heat, stirring. Scrape bottom of turkey pan and add drippings or giblets to gravy. Taste and adjust seasoning, then serve.