01 - Whisk together olive oil, lime juice, garlic, cumin, chili powder, smoked paprika, salt, and black pepper in a bowl. Add the sliced beef and toss to coat evenly. Let marinate for at least 15 minutes, or refrigerate for up to 2 hours for deeper flavor.
02 - Heat a large skillet or cast-iron pan over medium-high heat. Add the marinated beef in a single layer and sear for 2-3 minutes per side until browned and just cooked through. Remove beef from the pan and set aside, keeping warm.
03 - In the same pan, add the sliced bell peppers and onion. Sauté for 5-7 minutes, stirring occasionally, until vegetables are softened and slightly charred.
04 - Return the seared beef to the skillet and toss with the vegetables to combine. Cook for 1-2 minutes until everything is heated through.
05 - Heat the tortillas in a dry skillet for 30 seconds per side or microwave wrapped in damp paper towels for 20-30 seconds.
06 - Spoon the beef and vegetable mixture into warm tortillas. Top with fresh cilantro, a squeeze of lime, and your choice of salsa, guacamole, or sour cream.