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.
Andreas Viestad shares his tips with Epicurious: ·Lingonberry preserves can be found in some supermarkets and in specialty stores. Cranberries can be used as a substitute. ·Norwegian or other Scandinavian aquavit is available in most liquor stores. To make your own mock version, add the following spices to a 1-liter bottle of vodka: 2 teaspoons caraway seeds, 1 teaspoon fennel seeds, 2 teaspoons dill seeds, 2 star anise, 1 tablespoon coriander seeds, 1 whole clove, one 1-inch cinnamon stick (optional), and 2 teaspoons cumin seeds (optional). Let stand 2 to 3 weeks, shaking the bottle occasionally, then strain and discard the solids. This infused vodka can be substituted for aquavit in recipes, and is also delicious drunk straight.
Adapted with permission from Kitchen of Light: New Scandinavian Cooking by Andreas Viestad, (C) 2003 Artisan