Place the steak into a sealable bag. Place the peppers and onion into a separate sealable bag. Add the olive oil, lime juice, chili powder, ground cumin, cayenne pepper, salt, black pepper, and garlic to a jar with a screw top or tight fitting lid. Shake until well combined. Pour ⅓ of the marinade over steak, ⅓ of marinade over vegetables and reserve the remaining marinade in the jar for using when cooking the steak fajitas. Seal the bags tightly and refrigerate for one hour to overnight.
When ready to cook, heat a large skillet over medium-high heat. Pour the vegetables into the skillet and cook until just tender crisp, about 5 minutes. Remove the vegetables from the skillet to a plate and add steak strips to the same skillet. When the steak is cooked throughout (about 7-10 minutes), add vegetables back to the skillet along with the reserved marinade. Serve with warm tortillas and desired optional garnishes.
