Trim chicken of any excess fat. Season generously with salt and pepper. Heat oil and butter in a large, lidded sauté pan over medium-high flame. Working in batches, brown chicken on both sides until golden, about 3 or 4 minutes per side. Transfer to a plate.
Reduce heat to medium and add onion. Cook, stirring almost constantly, until just softened, about 2 minutes. Add garlic and cook, stirring constantly, for 45 seconds.
Return chicken to pan along with any juices, reduce heat to low and cook, covered, until chicken is cooked through, 25 to 30 minutes. Turn the chicken about halfway through and baste with pan juices. (Use an instant read thermometer to make sure the chicken is cooked to at least 165ºF.)
Meanwhile, crush the juniper berries with a mortar and pestle or the bottom of a heavy saucepan.
Flambé the chicken. When the chicken is cooked through, heat the gin in a small saucepan until hot. Turn off the heat (and your vent fan) and carefully light the gin with a long match. Pour it over the chicken and let the alcohol burn off (have a pan lid ready to cover the pan if the flames get too feisty—ours did not).
Sprinkle the juniper berries over the chicken and turn the chicken pieces over to coat them thoroughly with the sauces and juniper berries, adding 1 or 2 tablespoons of water if the pan seems too dry. Cook until sauce is slightly reduced, maybe 2 minutes.
Plate the chicken, spooning the sauces over it. Serve.