1. Rinse steak and pat dry; cut to fit large skillet. Combine juice, cumin, and garlic in a large heavy-duty resealable plastic bag. Add steak, season with salt and pepper, then seal bag and turn until steak is coated well. Marinate at room temperature, turning bag occasionally, 30 minutes.
2. Preheat broiler. Arrange tortillas in a single layer on oven rack 4 inches from heat and broil until slightly crisp, 30 seconds to 1 minute.
3. Heat 1 tablespoon oil in a large nonstick skillet over moderately high heat until hot but not smoking. Add bell peppers and onion; season with salt and pepper. Cook, stirring, until golden brown, about 6 minutes. Transfer to a serving platter. Remove steak from marinade (discard marinade). Heat remaining 1/2 tablespoon oil in skillet; add steak and cook, turning once, about 4 minutes (depending on thickness) for medium-rare. Transfer to cutting board and let stand 5 minutes. Cut into 1/4-inch-thick slices.
4. Arrange steak on platter with bell peppers and onion. Serve tortillas, salsa, Cheddar, and sour cream on the side. Makes 4 servings.