Combine garlic, jalapeno, cumin, cilantro, lime juice, sugar and oil in a mixing bowl. Whisk until well combined and transfer to a plastic gallon-size zip top bag, add flank steak and marinate under refrigeration for 1 to 4 hours.
Assemble tomatoes, cheese and lettuce leaves on a platter.
Preheat grill to medium-high heat and place 8-inch cast iron pan directly on the grill or over side burner set to medium-high heat.
Remove steak from marinade, pat dry and season with salt and pepper as desired. Grill steak for 6 to 8 minutes per side or to desired doneness. Remove and allow steak to rest 5 minutes before slicing into 1/2-inch thick slices.
Meanwhile, coat preheated cast iron pan with non-stick spray. Add corn, red onion and poblano peppers; saute for 6 to 8 minutes or until corn has nice char marks and peppers are tender crisp. At the same time, coat Mission(R) Soft Taco Flour Tortillas with non-stick spray and grill on each side no more than 30 seconds.
Immediately place sliced steak on top of corn mixture in cast iron pan and serve sizzling hot.