Heat oven to 350°F. Spray cookie sheet with nonstick cooking spray. Carefully unroll dough onto sprayed cookie sheet. Press or roll to form 14x12-inch rectangle.
In small bowl, combine mayonnaise and mustard; blend well. Spread over dough to within 1 inch of all edges. Layer half each of the pastrami and cheese lengthwise in 5-inch-wide strip down center of dough to within 1 inch of each short side, overlapping pastrami and cheese. Repeat layering with remaining pastrami and cheese. Sprinkle with onions.
Fold long sides of dough over filling; pinch edges in center to seal. Fold ends under 1 inch; seal. Cut several slits in top for steam to escape.
Bake at 350°F. for 26 to 30 minutes or until deep golden brown. Cool 5 minutes. Cut into crosswise slices.