Step 1 Combine stock, bay leaves, thyme, port, ginger, and jam in a large saucepan. Bring mixture to a boil. Reduce heat, and simmer until stock has reduced to 1 cup, about 1 hour. Remove from heat, strain, and transfer to a clean small saucepan. Set aside.
Step 2 Combine peppercorns, juniper berries, and rosemary in a spice grinder. Grind, allowing some texture to remain.
Step 3 Heat oven to 350 degrees. Place a large roasting pan in the oven. Salt both sides of each rack well, and rub the ground spice mixture into the meat.
Step 4 Heat olive oil and 1 tablespoon butter in a large skillet set over medium-high heat. Place both racks in skillet, and brown each side, about 2 minutes per side, using tongs to maneuver racks in skillet. Transfer racks to the preheated roasting pan.
Step 5 Roast venison 30 to 35 minutes for medium rare. Remove roasting pan from oven, and transfer meat to a cutting board to rest 15 minutes.
Step 6 Return sauce to a boil. In a small bowl, combine remaining 2 teaspoons butter with flour; mix until a paste forms. Reduce heat, stir in cranberries; let simmer until berries are soft and sauce is glossy. Whisk in the butter mixture. Serve the red-currant-and-cranberry sauce with the venison.