Prepare the marinade by combining the orange juice, lime juice, garlic, olive oil, salt, and cumin in a bowl. Whisk ingredients together until smooth.
Cut the steak into strips about 1/4-inch thick. Add the steak to the marinade, ensuring each piece is well-coated. Cover and refrigerate for at least 1 hour.
Preheat a heavy cast-iron skillet or grill pan over medium-high heat. Add just enough oil to coat the base of the pan and sear the steak strips in batches for 1 to 2 minutes on each side until charred and caramelized.
Warm the tortillas in a dry skillet or oven until pliable and lightly toasted.
In a small bowl, toss the chopped cilantro and red onion together to create a fresh topping.
To assemble, place the grilled steak on warm tortillas, top with the cilantro-onion mixture, and add a squeeze of lime juice and optional red pepper flakes for heat.