Pat the meat dry with paper towels. Combine 6 of the juniper berries, the fennel seeds, 1 teaspoon salt, and 1 teaspoon pepper. Rub the meat with the spices and place it on a plate. Set aside at room temperature while you make the sauce.
Put the flour in a small bowl and whisk in 1/4 cup of the stock; make sure there are no lumps. Pour into a small saucepan, add another 1/4 cup stock, and bring to a boil, whisking constantly. When the mixture has started to thicken, stir in the remaining 1/2 cup stock and bring to a boil. Add the sour cream and the remaining 2 juniper berries, reduce the heat, and simmer for 2 to 3 minutes. Add the brown cheese and stir until melted and incorporated. Set the sauce aside.
Heat the butter in a cast-iron or other heavy skillet over high heat. Sear the fillets for 5 minutes, turning to brown on all sides. Transfer the meat to a plate and let rest for 4 to 5 minutes. Return the meat to the skillet and cook for 3 to 4 more minutes, until medium-rare. Let rest for 5 minutes.
Meanwhile, add the aquavit to the sauce and bring to a simmer over medium heat. Season with salt and pepper to taste. Cut the fillets into 1/2-inch slices and place on four plates. Drizzle the sauce over the meat. Serve immediately, with lingonberry preserves on the side.