1. In a shallow glass dish, stir together garlic, lime juice and chili powder. Add steak, turn to coat and let stand for 5 minutes, turning occasionally.
2. In a large nonstick or seasoned cast-iron skillet, warm 1 Tbsp. oil over medium-high heat. Cook bell peppers, stirring occasionally, for 4 minutes. Add cumin seeds; continue to sauté until bell peppers are lightly browned and tender, about 2 minutes more. Transfer to a platter.
3. Add remaining 1 Tbsp. oil to skillet. Add steak and cook, turning once, until browned, about 6 minutes for medium-rare. Transfer to a cutting board and let stand for 5 minutes before slicing thinly against the grain.
4. Top tortillas with bell peppers and steak. Serve with additional toppings such as sour cream, avocado, cilantro and lime wedges, if desired.