Flat iron steak has become my favorite versatile cut of beef. It cooks quickly and has the rich flavor of more expensive steaks. The grilled vegetables and cheese make this taco irresitable! We rarely have any leftover, which is a shame because the filling makes for an amazing omelet!
Combine salsa verde, lime juice and chopped cilantro in a large zip top bag. Add flat iron steak and seal. Marinate for 2 hours, turning bag once during marinating.
Preheat grill, (I use gas), to medium high heat.
Remove steak from marinate and place on grill. Cook 2-3 over medium high heat and reduce heat to medium and cook 6-8 minutes per side for medium to medium rare doneness. Remove from grill and allow to rest 15 minutes.
Toss tomatillos and red onion slices with olive oil and kosher salt, place on grill. Grill, turning frequently. Onions will take 4-6 minutes total cooking time. Tomatillos will take 10-12 minutes cooking time.
Spread tortillas onto grill, turn after one minute, grill an additional minute, remove.
Slice tomatillos into wedges. Slice steak thinly against the grain of the meat.
Assemble tacos by layering onto a grilled tortilla; 2-3 slices of steak, a ring or two of onion, 3-4 tomatillo wedges, a few thin slices of avocado and crumbles of queso fresco. Finish with a squeeze of fresh lime juice.