Clean the animal (amputate the neck, giblets and rump) and secure the liver. Stir the balsamic vinegar, 2/3 of the oil, the mustard, salt and pepper to make a vinaigrette-like soup, finally add the cornstarch and 2 cloves of pressed garlic. Stir again heartily. There should be a certain viscosity, because now the bird is brushed in&out with it and left well with closed lid for at least 5 hours to itself.
Now open the wine and pour a glass for the cook. In a small frying pan, heat the butter with the sage leaves over medium heat until just flat yet not burning. Pour in the chicken livers and sizzle for 1 minute on each side. If desired salt & pepper on it form, cut a slice of fresh bread into small pieces and polish off the bread, liver and wine with relish.
On it at the beginning prepare the remaining garlic cloves, with fresh garlic it should not be a problem to unpack the parts intact. Nothing is cut into small pieces or pressed here, the stuff is a vegetable this time, not a spice. Preheat the stove. If you have the time to check every 5-10 minutes, set it to 225, otherwise to 180 degrees.
While the oven is heating up, clean the leeks and cut them into rings.
Contrary to all appearances, the garlic plume on the following day is quite limited, because only two cloves are really disassembled, with the rest you have a good taste oh