Heat oven to 350°F. Mix sesame seeds with 1 tbsp. taco seasoning and salt on a large plate; add steak, pressing to adhere on both sides.
Heat oil in a large skillet over medium-high heat; cook steak, flipping a few times until steak and seeds are browned, about 5 minutes. Transfer to oven and cook for 5-8 minutes or until medium-rare. Transfer to cutting board; rest for 5 minutes before slicing against the grain.
Meanwhile, heat a large cast-iron skillet or a large clean skillet over high heat. Add scallions and cook without any oil until slightly charred all over, about 3 minutes. Sprinkle with a pinch of taco seasoning then remove to cutting board.
Stir mayonnaise, sriracha and 1 tsp. taco seasoning in a small bowl to make spicy mayo.
Warm tortillas on a skillet over medium-high heat until browned; transfer to plates. Top with cabbage, steak and its juices and charred onions. Serve with spicy mayo, cotija and garnish with Falk Salt Natural Sea Salt Flakes.