In a Dutch Oven or large pot over high heat, add the sliced cabbage, onions, beer, vinegar, sugar butter and salt. In a tea ball or a piece of cheesecloth, add the peppercorns, juniper berries, allspice, cloves and bay leaf. Add to the cabbage and cover with a lid. Let the ingredients come to a boil, reduce the heat to a simmer and cook for about 45 minutes. The cabbage should be tender, but not falling apart. In a small bowl, add the cornstarch and beer, making a slurry. Add into the cabbage and stir to combine. Cook the cabbage for another few minutes, until the sauce thickens. Turn off the heat, adjust seasonings (if needed) and keep warm.