Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.
Heat the olive oil in a large skillet over medium heat.
Add the Italian sausage and cook for 6–8 minutes, breaking it into crumbles until browned.
Stir in the minced garlic, Italian seasoning, and red pepper flakes. Cook for 1 minute, then remove from the heat and let cool slightly.
In a large bowl, combine the cooked sausage, mozzarella cheese, Parmesan cheese, and chopped parsley.
Roll the pizza dough into a rectangle about 12 × 16 inches.
Spread the sausage mixture evenly down the center of the dough, leaving a 1-inch border around the edges.
Fold the sides of the dough over the filling and pinch the seams tightly to seal.
Place the loaf seam-side down on the prepared baking sheet.
Brush the top with the beaten egg and sprinkle with sesame seeds or Italian seasoning if desired.
Bake for 30–35 minutes, or until the bread is golden brown and cooked through.
Remove from the oven and let the bread rest for 15 minutes before slicing to help the cheese set.
Slice and serve warm with marinara sauce if desired.