Rinse the quail under cool running water, dry on paper towels, and season liberally with salt and pepper. Stuff half an onion into the cavity of each bird, and truss each by tying together the drumsticks with kitchen twine.

In a Dutch oven over medium high heat, warm the olive oil to just below smoking. Gently add the quail, and crisp on each breast side, about 2 minutes per side; then brown the back as well. Remove the quail from the pot and set aside.

Using the same Dutch oven, reduce the heat to medium, add the leeks, and cook until they begin to soften, stirring frequently, about 5 minutes. Add the dates, bouquet garni, and cider. Reduce the cider down for about 3 minutes, then add the stock and the quail. Let the liquid come almost to a boil, cover, and reduce the heat so the cooking liquid barely simmers. Cook until the quail are done, 15 to 20 minutes.

Remove the quail to a platter and cover with heavy-duty aluminum foil to keep them warm. Raise the heat under the Dutch oven and simmer briskly to reduce the cooking liquid until slightly thickened, 3 to 5 minutes. Spoon this sauce over the quail and serve.