With the rack in the middle position, preheat the oven to 200 °C (400 °F).
Sprinkle pork loin with juniper berries. Season with salt and pepper. In a large ovenproof skillet, brown meat in butter.
Roast in the oven for about 45 minutes or until a thermometer inserted in the centre of the meat reads 57 °C (135 °F). Place meat on a serving platter and cover with aluminum foil. Let rest until the thermometer reads 63 °C (145 °F).
In a small saucepan, bring broth, cream, tarragon sprigs and shallot and reduce by half. Meanwhile, in a small bowl, dissolve cornstarch in water. Add to reduced sauce, whisking constantly. Strain. Add fresh tarragon.
Slice meat and drizzle with sauce. Serve with Roasted Onions and Fingerling Potatoes (see recipe).