Using butcher's twine, tie loin at 1-in. intervals and transfer it to a baking sheet fitted with a metal cooling rack. In a small bowl, combine orange zest, thyme, rosemary, sage, 1/2 cup parsley, juniper berries, and 1 tsp. garlic and pat all over loin. Refrigerate at least 2 hours and up to 1 day.
Preheat oven to 350°. Roast loin on middle rack of oven until rare, 30 to 40 minutes (125° on a meat thermometer). Remove from oven and let rest, covered with foil, 15 minutes.
Melt 1 tbsp. butter in a large frying pan over medium heat. Add mushrooms and cook until they have stopped giving off liquid and start to brown. Using a wooden spoon, move mushrooms to side of pan. In center of pan, melt remaining 1 tbsp. butter. Quickly stir in remaining 1 tsp. garlic and 2 tbsp. parsley, then combine with mushrooms, stirring occasionally and being careful not to burn the garlic. Stir in cream and salt to taste.
Remove twine and lightly salt meat. Slice very thin and serve with mushrooms.
Note: Nutrition analysis is per serving.