Heat grill (or oven) to 450 degrees. Bake flatbread 3-5 minutes, until it begins to crisp.
Spread hummus onto flatbread. Layer arugula (reserving 3-4 leaves to garnish), steak, and crumbled blue cheese. Top with onion, if desired.
Place on grill, close lid and cook 3-5 minutes (if using oven, broil approximately 2 minutes). Add salt and pepper before serving.