Mix together marinade for steak in a small bowl. Add marinade and steaks to a large plastic bag and set aside, marinating for at least 15 minutes.
Meanwhile, prepare pickled red onions. Stir sugar, salt, garlic and peppercorns in with vinegar in a medium-sized mason jar. Place thinly sliced red onions in a strainer in the sink. Slowly pour a kettle full of boiling water over onions. Immerse semi-cooked onions in vinegar mixture and let sit for 10-15 min before serving. Onions will last in the fridge for a couple weeks - use them as a condiment for other meals!
Make citrus slaw by tossing all ingredients together. Meanwhile, preheat BBQ to med-high heat.
Add steak and corn tortillas to grill. BBQ steak for 2-3 min each side for medium-rare, and 4-5 min for well-done. Grill corn tortillas for about 1-2 min per side, longer if you want the taco shells to be crispy.
Add steak, coleslaw, pickled red onion, jalapenos, cilantro and sesame seeds to each taco. Serve and enjoy!