Red Wine Sauce: In large saucepan, brown shallots, mushrooms, and ground lamb in butter. Stir in red wine, chicken stock, and veal stock. Boil until reduced by half. Add juniper berries, bay leaf, thyme, garlic, and vinegar. Boil 10 minutes. Strain sauce, discarding solids. Return sauce to saucepan. Stir in lemon juice and salt and pepper to taste. Cover, keep warm.

Nut Stuffing: While sauce is reducing, in food processor, pulse pistachio nuts with on/off turns until chopped. Add chestnut puree, ground lamb, and brandy. Process briefly until blended.

Lamb Loin in Puff Pastry: Let puff pastry stand at room temperature according to package directions. Mound 1/4 of stuffing on each lamb loin. Using hands, mold stuffing around lamb until loin is completely enclosed with stuffing mixture. Preheat oven to 400ø. Gently unfold 1 sheet of pastry on lightly floured surface. Press pastry together at the two fold seams to seal. Cut pastry in half. Repeat with remaining sheet of pastry. Roll each pastry to 10x8" rectangle. Arrange a wrapped lamb loin crosswise in center of each sheet of pastry.

Overlap long sides of pastry over meat and fold over together to seal.