Place sliced onion strips into a heat-safe bowl and set aside. Mix the vinegar, water, salt, pepper, and juniper berries in a small pot and bring to a boil. Once boiling, pour hot liquid over the sliced onions. Cover the mixture and allow to marinate at room temperature for about one hour.
The pickled onions will stay fresh for about one month in the refrigerator and make an excellent topping for tacos or sandwiches.