Apple Walnut Strudel with Vanilla Sauce

5 Granny Smith or Pippin apples, peeled, cored and sliced
4 ounces butter
1/2 cup packed brown sugar
1/4 cup granulated sugar
2 teaspoons ground cinnamon
2 teaspoons lemon zest
1/2 cup raisins
1 tablespoon apple juice or water
1/2 cup walnuts, chopped
2 (8 x 12-inch) puff pastry dough sheets
Egg wash (1 beaten egg with 1 ounce water)
Sweetened whipped cream (optional)
Vanilla Sauce

In a large saut