Pound your flank steak thin. Add olive oil to pan on medium high heat and toss in onions until translucent and tender. Next add garlic and spinach until spinach is slightly wilted.
Take spinach mixture off the heat and spread across the center of the flank steak along with the sliced red peppers. Roll the flank steak and tie with string. Rub the spice rub all over the outside of the rolled steak.
Sear flank steak for 2 minutes on each side then leave on the grill in indirect heat for 30 minutes rotating every 5 minutes.
Pull meat off the grill and let rest for 10 minutes before slicing and serving.