Preheat your oven to 375°F (190°C).
In a skillet, heat olive oil over medium-high heat. Add the diced onion and bell pepper.
Season with garlic powder, salt, and pepper, then cook until the vegetables are tender, about 5-7 minutes.
Add the thinly sliced ribeye steak to the skillet, cooking until browned and fully cooked, about 4-5 minutes.
Remove the skillet from heat and stir in the shredded mozzarella cheese until melted and well combined.
Roll out the pizza dough on a lightly floured surface into a rectangle, about 12x16 inches.
Spread the cheesesteak mixture evenly down the center of the dough.
Carefully roll the dough over the filling, tucking it as you go, and pinch the ends to seal.
Place the stromboli seam-side down on a parchment-lined baking sheet, then brush with the beaten egg.
Bake in the preheated oven for 25-30 minutes, or until golden brown.
Let the stromboli cool for a few minutes before slicing and serving.