Set a heavy skillet over medium-high heat, add a drizzle of oil and cook the steak for 4-5 minutes per side, or until cooked to your liking.
Set aside on a cutting board to rest, then thinly slice. (Alternatively, put the steak into a slow cooker with 1/3 cup of water, cover and cook on low for 6 hours. Shred the meat, tossing with the sauce.)
Toss the sliced cabbage with the sesame oil and a pinch of salt.
Whisk together the mayo and Sriracha.
Serve the sliced or shredded meat on the corn tortillas, topped with cabbage and Sriracha mayo, with avocado and cilantro, if you like.