Combine Mojo Criollo, lime juice, garlic, cumin, salt, pepper, oil, and cilantro in a large resealable bag. Shake to mix well.
Pat the flap meat dry with paper towels and cut into manageable sections of about 4-5 inches long for even cooking.
Add the meat to the marinade, seal the bag pressing out excess air, and massage to coat evenly. Refrigerate for at least 4 hours or overnight (8-12 hours). Do not exceed 24 hours.
Remove marinated meat from the refrigerator and let it rest at room temperature for 20-30 minutes before cooking.
Heat a cast iron skillet over medium-high heat until very hot. Add a thin layer of vegetable oil and heat until shimmering.
Shake excess marinade from the meat and place in a single layer in the hot skillet. Cook without moving for 3-4 minutes per side until a deep brown, caramelized crust forms.
Check doneness with a meat thermometer: 130°F (54°C) for medium-rare, 140°F (60°C) for medium, or 150°F (65°C) for medium-well.
Transfer cooked meat to a cutting board and tent loosely with foil. Rest for 5 minutes to allow juices to redistribute.
Slice or chop the meat into small, bite-sized pieces against the grain for maximum tenderness.
Heat a dry skillet over medium heat. Warm each corn tortilla for 20-30 seconds per side until softened and lightly charred. Stack under a clean towel to keep warm.
Place a generous portion of chopped carne asada onto each warm tortilla. Top with diced white onion, fresh cilantro, and a squeeze of lime. Serve with salsa on the side.