Heat oven to 350°F. Grease large cookie sheet.
If using crescent rolls: Unroll both cans of dough into 2 long rectangles. Place dough rectangles with long sides together on cookie sheet, forming 15x10-inch rectangle. Press edges and perforations to seal. If using dough sheets: Unroll both cans of dough. Place dough rectangles with long sides together on cookie sheet, forming 15x10-inch rectangle. Press edges to seal.
Place spinach lengthwise in 4-inch-wide strip down center of dough to within 1/2 inch of each end. Top with half of the cheese, the roasted peppers and prosciutto. Spread pesto over prosciutto. Top with remaining cheese. Bring long sides of dough up over filling, overlapping 1 inch; press edges and ends to seal.
In small bowl, beat egg yolk and water until well blended. Brush over top of dough.
Bake 25 to 35 minutes or until golden brown. Cut into crosswise slices.