Combine all marinated beef ingredients in a ziplock bag. Seal and massage the bag gently to evenly coat the steak in the marinade. Place in the refrigerator and allow to marinate for at least 2 hours, or overnight for deeper flavor.
Take the steak out of the refrigerator 30 minutes before cooking to bring it to room temperature. Remove from the marinade and pat dry with paper towels. Discard the used marinade.
Drizzle about ½ tablespoon of oil on each side of the steak, using a total of 1 tablespoon. Preheat a BBQ grill or heavy-bottomed skillet over high heat. Once hot, cook the beef to your preferred doneness, searing each side for maximum flavor.
Transfer the cooked steak to a plate and loosely cover with foil to rest. This helps retain the juices.
Return the skillet to high heat and add ½ tablespoon of oil. Toss in the sliced onions, season with salt and pepper, and sauté until softened and charred at the edges. Transfer to a serving plate.
Add the remaining ½ tablespoon of oil to the skillet. Add sliced bell peppers, season with salt and pepper, and cook until they develop a nice char while still retaining a bit of crunch. Remove and place alongside the onions.
Slice the rested beef thinly, cutting against the grain to maximize tenderness.
Serve the sliced steak with warm tortillas and your choice of toppings such as avocado, fresh cilantro, sour cream, lime wedges, and fresh tomatoes or pico de gallo for added flavor and texture.