Combine the juniper berries, thyme, and the pepper in a small mixing bowl. Rub the lamb with the spices, cover, and refrigerate for 2 hours.
Preheat the oven to 400 degrees F.
Heat the oil and butter and brown the meat on all sides in a large skillet. Place the meat fat-side up in a roasting pan. Arrange the carrots, onions, bay leaf, and salt around the meat. Pour 1 cup of wine over it and place in the oven. Baste frequently with the remaining 1/4 cup wine and water if it becomes too dry. Roast for 1 1/2 hours. Check for doneness with a thermometer: 140 degrees F for rare, 150 to 155 degrees for medium, and 160 degrees for medium-well.
Turn off the oven. Remove the meat from the pan, transfer it to a large platter and let it sit in a warm oven for 10 minutes with the oven door open.
Deglaze the pan with 1/2 cup hot water, or more, if necessary. Degrease and strain the gravy into a small saucepan. Whisk in the sour cream. Dissolve the flour in 1 to 2 tablespoons water and add it to the sauce. Simmer for a few minutes until the sauce thickens. Season with salt and pepper. Slice the meat and place it on the serving platter. Serve the gravy separately.