Roast the thighs as directed (using a pot large enough to hold the cabbage, apples, and thighs. When the thighs are about 30 minutes away from being done, pour out all but about 3 tablespoons of the fat, and add the cabbage and crushed juniper berries and cook, covered, until shrunk by half, about 20 minutes. Add the apples and vinegar. Cover the pan and continue to cook for 30 minutes more.
Before serving, season the cabbage and apples with salt and pepper and a little more vinegar if the mixture lacks tang. Arrange the thighs on top of the cabbage and apples to serve.