Combine first 6 ingredients in a large zipper-style plastic bag, mixing well. Seal and set aside.
To butterfly steak, use a sharp knife to cut the steak in half lengthwise, cutting almost to, but not completely through, the opposite side, to form a butterfly shape. Add the butterflied steak to the marinade in bag; seal bag securely, removing as much air as possible, and place in a large bowl. Marinate for 8 hours or overnight in refrigerator, turning occasionally.
Preheat oven to 350° F.
Remove steak from marinade, discarding marinade. Spread spinach over steak to within 1/2-inch of edges; top with red peppers and onion.
Combine chilies, garlic, cumin, and chili powder; sprinkle chili mixture evenly over pepper and onion layer. Sprinkle evenly with cheese.
Roll steak, starting at short side. Secure with heavy string at 2-inch intervals. Place in a shallow roasting pan.
Bake for 45 minutes. Let stand for 5 minutes before slicing to serve.
Servings: 6 to 8