Saute chicken livers in 3 tablespoon butter until cooked. Set aside
Heat 1 tablespoon butter in skillet over medium-high heat. Cook mushrooms and onion until vegetables are tender and liquid is evaporated.
In an electric blender puree the chicken livers, brandy, mushroom-onion mixture, salt and pepper.
Precook the beef until browned, approx 4 minutes per side, (do not cook to desired doneness). Unfold pastry on lightly floured surface. Roll into rectangles 2" longer and 3" wider than beef. Brush with egg mixture. Spoon mushroom mixture onto pastry to within 1" of edges. Place beef in center of mushroom mixture. Starting at long sides, fold pastry over beef. Fold in ends. Press edges to seal. Place seam-side down on baking sheet. Brush with egg mixture. Bake in a 350º oven for 20 min. or until golden brown