Cut off roots and trim leaf ends of leek. Separate leaves of leek; rinse under cold water to remove all sand. Then cut leek crosswise into 1/2 inch slices. Coarsely chop apple and slice mushrooms. Set aside.
In a 10-inch skillet over high heat, cook ground beef and curry powder, stirring frequently until ground beef just looses its pink color. With slotted spoon remove ground beef mixture to a large bowl.
In drippings remaining in skillet over medium heat, add 1 tablespoon oil if necessary. Cook leek, apple, and mushrooms until tender, stirring occasionally. Remove leek mixture to bowl with beef mixture.
Into drippings remaining in skillet, stir 1 tablespoon oil and flour until blended. Gradually stir in salt, bouillon, pepper, and water; cook, stirring until mixture boils and thickens; boil for 1 minute. Stir sauce and currants or raisins into beef mixture; set aside and cool. (you may want to place beef mixture into refrigerator to help it cool off, or you can make it a day ahead and proceed the next day).
While meat mixture is cooling (or next day), thaw pastry as label directs (refer to note below).
Preheat oven to 375 degrees fahrenheit. Unfold pastry onto lightly floured surface; with floured rolling pin, roll pastry into 12" x 10x" rectangle. Place pastry onto a greased jelly roll pan. Spread beef filling in a 3" wide strip lengthwise down center. Cut pastry on both sides of filling crosswise into 1 1/2" wide strips to within 1/2" of filling. With pastry brush, brush edges of pastry with water. Place strips at an angle across filling, alternating sides for braided effect and making sure that end of each strip is covered by the next strip so strips stay in place as pastry bakes. Pinch last strip to bottom of braid to seal.
Bake braid 25 - 30 minutes until pastry is golden and meat mixture is hot.
To serve, cut into slices.
NOTE: In order for puff pastry to keep it's puffiness, always keep it on the cold side. Therefore, it is vital to place your meat mixture when cold on the puff pastry. If you are planning on baking the beef braid at a later time, keep braid in the refrigerator and place it into the oven immediately upon removal from the refrigerator.