Make the Marinade: Toss garlic, both citrus juices (and that orange zest), soy sauce, onion, jalapeño, all the Mexican spices, and avocado oil into your food processor. Blend until it looks like it might actually work out. Pour this marinade over your flank steak and let it marinate in the fridge for at least 2 hours. Overnight marinating is even better: patience pays off here.
Grill the Steak: Heat your grill to "aggressively hot" and cook the marinated steak 2-3 minutes per side. You want char marks that look like they mean business. Don't move it around: let it do its thing.
Rest and Slice: Let the grilled steak rest for 5 minutes (this isn't optional), then slice against the grain. This is the difference between tender bites and jaw workout sessions.
Assemble the Tacos: Heat your flour tortillas on an oiled griddle or skillet. Layer with Mexican cheese first (it's the glue that holds dreams together), add sliced steak, fold over, and let each side get crispy and golden. The cheese should be properly melted and maybe slightly rebellious around the edges.
Add Toppings: Hit them with fresh salsa, homemade guacamole, cilantro, and whatever else your heart desires.