Martini Chicken

In an electric coffee/spice grinder finely grind juniper berries. In a small bowl mix the ground juniper with the salt. Arrange chicken breast slices on a rimmed baking sheet and sprinkle juniper salt evenly on both sides of each piece of chicken, patting it on.
Heat olive oil with 1 tablespoon butter in a 12-inch heavy skillet over medium-high heat until foam subsides. Cook chicken in batches (don't crowd pan), turning once or twice (adjust heat lower as necessary to keep bits on bottom from burning) until just cooked through, 2 to 4 minutes total per batch. Transfer chicken as cooked to a platter.
Have a lid for the skillet ready. Add olives and onions to skillet and saute over medium-high heat, stirring, 30 seconds. Add vermouth, stirring and scraping up browned bits, then carefully add gin. When hot, the gin will likely ignite, so keep a safe distance from the pan. If the gin does ignite, cover the pan with the lid. When the flames die out, remove the lid, then boil the liquid over medium-high heat until it is reduced to about 1/3 cup. Add any juices that have accumulated on the platter and return the liquid to a simmer.
Remove the pan from the heat and add the remaining 3 tablespoons butter, cut into tablespoon pieces, then swirl the pan until the butter is incorporated. Spoon the sauce over the chicken and season with the pepper. Let each diner squeeze a bit of lemon juice to taste over their serving, if desired.