Thaw and/or cook prepared meatballs according to package or recipe directions. Chop the meatballs into 1/2-inch pieces. Toss the chopped meatballs in 2/3 cup of the pizza sauce. Set aside or refrigerate until needed.
If making ahead and refrigerating, warm the meatballs slightly before adding to the dough so the cold won't affect the internal baking. Thaw or rise pizza dough according to package directions.
Preheat oven to 375°. Line a large rimmed baking sheet with parchment paper and set aside.
Roll the dough into a 12 by 10-inch rectangle on a lightly floured counter with the long side parallel to the countertop edge. The short sides will be on your left and right.
Layer the provolone cheese evenly over the dough leaving a 1/2-inch border along the top and sides. Distribute the chopped meatballs over the provolone, sprinkle evenly with the green peppers and crushed red pepper flakes, then top with the shredded mozzarella. Brush the top and side borders of the dough with the lightly beaten egg.
Reserve remaining egg to brush on finished stromboli before baking.
Fold the bottom third of the stromboli in toward the middle, like a business letter. Carefully fold the top third of the stromboli down to cover the first fold, creating a log. Pinch the seams firmly to seal.
Transfer the stromboli to the prepared baking sheet, seam side down. Pinch the ends firmly together to seal and tuck underneath. Allow the stromboli to rise in a warm location for 1 hour.
Brush the top of the stromboli with the remaining beaten egg. Using a very sharp large knife, make 6 evenly spaced 1/2-inch-deep slashes, 2-inches long, on top of the stromboli.
Sprinkle with sesame seeds and bake until the crust is golden brown and the center registers 200° on an instant read thermometer, about 35 to 40 minutes. Rotate the pan halfway through baking and tent the stromboli with foil if browning too quickly.
Transfer to a wire rack and cool for 10-minutes.
Cut into 2-inch thick slices and serve with remaining pizza sauce and grated Parmesan cheese if desired.