In a medium saucepan, heat 1 teaspoon of the oil. Add the whole garlic cloves and cook over low heat, stirring, until golden and fragrant, 5 minutes. Add 4 cups of water and bring to a boil. Cover and simmer over low heat until reduced to
2 cups, about 1 hour. Strain the garlic broth into a bowl.
In another bowl, whisk 1 cup of the garlic broth with the sour cream and porcini and season with salt; reserve the remaining garlic broth for another use.
Preheat the oven to 350°. In a large cast-iron skillet, heat the remaining 1 tablespoon of oil. Season the chicken with salt and cook over moderate heat until golden all over, about 10 minutes total.
Transfer the chicken to a plate. Pour off all but 1 tablespoon of the oil in the skillet. Arrange the potato slices in the pan, overlapping them slightly. Set the chicken skin side down on top of the potatoes. Scatter the cherries and crushed garlic around the chicken and pour the garlic sauce with porcini on top.
Roast the chicken for 20 minutes. Reduce the oven temperature to 300° and roast for about 45 minutes longer, until the potatoes are tender and the chicken is cooked through.
Preheat the broiler and arrange the rack 6 inches from the heat. Turn the chicken skin side up and broil until
the skin is golden and crisp, about 8 minutes. Garnish with celery leaves and serve.