In a large skillet cook potatoes in the 1 tablespoon oil over medium heat for 5 minutes or until tender; set aside.
Meanwhile, heat broiler. Place flatbreads on an extra-large baking sheet; lightly brush bread tops with oil. Top with half of the cheese. Broil 3 to 4 inches from the heat for 2 to 3 minutes or until cheese begins to melt. Top barely melted cheese with spinach, roast beef, potatoes, and remaining cheese.
Broil for 3 to 5 minutes more or until heated through. Drizzle with additional olive oil, then, if desired, top with pepper slices and crushed red pepper.