Heat grill to medium-high. Season the steak with the cumin, ½ teaspoon salt, and ¼ teaspoon pepper.
In a large bowl, toss the bell pepper, oil, and ¼ teaspoon each salt and pepper. Set aside.
Grill the steak, 4 to 5 minutes per side for medium-rare. Let rest for 5 minutes before slicing.
Meanwhile, grill the peppers, turning occasionally, until tender, 8 to 10 minutes.
Fill the tortillas with the steak, peppers, and avocado. Serve with the sour cream and hot sauce, if desired.