In a large heavy-bottomed stock pot over moderate flame, cook the sliced onions and garlic in olive oil until jam-tender and light brown, about 20 minutes, stirring frequently. Add flour and stir well. Add wine, stir well, and simmer 5 minutes. Add turkey stock and bring to a boil, reduce flame to simmer, and continue cooking until reduced by half. Strain the sauce, season with salt and pepper, skim off any accumulated fat, and keep the sauce warm. Just before serving, whisk in a few tablespoons of strained pan juices from the turkey, taste, and adjust the seasoning, if necessary.