Poach the parboiled garlic cloves for about 40 minutes in the stock, covered, kept at a slight simmer.
Color the seasoned chicken legs in butter over medium heat (20 to 25 minutes) and transfer them to an oven casserole. Strain the stock to remove the garlic cloves, taking care not to damage them. Scatter the garlic over the chicken pieces, distribute the lemon slices, and put the casserole aside, covered, until the sauce is prepared.
Remove any excess fat from the pan in which the chicken was browned (leaving just enough to absorb the flour), add the flour and cook, stirring, over low heat for a few moments. Deglaze with the white wine over high heat, stirring and scraping with a wooden spoon, add the stock, and pour into a small saucepan; this is important, the small surface permitting a more rapid and complete skimming and degreasing of the sauce while preventing, at the same time, an exaggerated reduction. Skim (skin) for about 15 minutes, removing any traces of loose fat from the surface with absorbent paper. Pour the sauce over the chicken and its garnish and cook, covered, in a 375° to 400° oven for 40 to 45 minutes. The lemon will have almost completely disappeared into the sauce; the garlic cloves should be absolutely intact with a consistency of melting purée; the sauce must be tasted to be believed.