In a small bowl, whisk together orange juice, lime juice, minced garlic, soy sauce, vegetable oil, cumin, and oregano. Place the steak in a gallon zip-top bag or shallow dish and pour the marinade over it, turning to coat. Seal and refrigerate for at least 2 hours, preferably 4 hours or overnight.

Remove the steak from the refrigerator 30 minutes before cooking. Heat your grill or grill pan to high heat, about 450-500F. Scrape off excess marinade and pat the steak dry with paper towels - this helps it char properly.

Grill the steak for 3-4 minutes per side for flank steak (2-3 minutes for thinner skirt steak), until deeply browned with good char marks and an internal temperature of 130F for medium-rare. Transfer to a cutting board and let rest for 5-10 minutes - this is crucial for juicy meat.

Find the direction of the muscle fibers and slice thinly across them, not with them. For flank steak, this means cutting crosswise into strips. Pile the sliced meat onto a warm plate.

Quickly char the corn tortillas directly over a gas burner or in a dry skillet for 20-30 seconds per side until soft and slightly blistered. Wrap in a clean kitchen towel to keep warm.

Layer each tortilla with a generous handful of steak, a spoonful of diced onion, a sprinkle of cilantro, and your favorite salsa. Squeeze fresh lime over the top and serve immediately.