Marinate the Beef:
In a small bowl, whisk together olive oil, lime juice, orange juice, chili powder, cumin, paprika, garlic powder, oregano, salt, and pepper.
Place the beef in a resealable bag or shallow dish and pour the marinade over it. Toss to coat. Cover and refrigerate for at least 30 minutes, or up to 8 hours for deeper flavor.
Cook the Beef:
Heat a skillet or grill pan over medium-high heat. If using steak, remove it from the marinade and cook for 4-5 minutes per side until medium-rare or desired doneness. For ground beef, cook until browned and fully cooked, breaking it up into crumbles.
Warm the Tortillas:
Heat the tortillas in a dry skillet, directly over a gas flame, or in the oven until warm and slightly charred. Wrap them in a clean kitchen towel to keep warm.
Assemble the Tacos:
Slice the steak thinly against the grain or spoon the cooked ground beef into each tortilla.
Top with diced onion, chopped cilantro, and a squeeze of lime juice. Add optional toppings like avocado, cheese, or salsa as desired.
Serve:
Arrange the tacos on a platter with lime wedges and extra toppings on the side. Enjoy immediately!