Combine pickled onion ingredients in medium bowl; stir well. Cover and refrigerate while preparing beef.
Combine seasoning ingredients in small bowl; press evenly onto beef steaks. Cover and refrigerate 15 minutes to 2 hours.
Preheat charcoal grill to medium heat.
Place steaks on grill over medium, ash-covered coals. Grill, covered, 10 to 14 minutes for medium-rare to medium doneness, turning once. Remove from grill; let stand 5 minutes.
Meanwhile drain pickled onions well. Carve steaks into thin strips. Top tortillas evenly with steak, queso fresco and pickled onions. Sprinkle with cilantro, as desired.
Cook's Tip: To prepare on gas grill, preheat grill according to manufacturer's directions for medium heat. Grill steaks, covered, 12 to 16 minutes for medium-rare to medium doneness, turning occasionally.
Cook's Tip: 1 pound boneless beef top sirloin steak, cut 3/4-inch thick, may be substituted for shoulder top blade (flat iron) steaks. Grill, uncovered, over medium-ash covered coals 13 to 16 minutes (over medium heat on preheated gas grill, covered, 8 to 13 minutes) for medium rare to medium doneness, turning occasionally.
Cook's Tip: Pickled onions may be prepared ahead and refrigerated as long as overnight, if desired.
Cook's Tip: Adobo seasoning is a spice blend used in Latin American cooking and usually contains garlic, onion, black pepper, salt, oregano, cumin and ground red pepper. If unavailable, a blend of 1 teaspoon seasoned salt, 1/4 teaspoon garlic powder and 1/8 teaspoon dried oregano leaves, crushed, may be substituted.
Cook's Tip: 1 tablespoon chopped chipotle pepper in adobo sauce may be substituted for ground chipotle pepper. Regular sweet or hot paprika and chili powder may be substituted for smoked paprika and ground chipotle chile pepper, but there will be no smoky flavor.
Cook's Tip: Queso fresco is a white, slightly salty fresh Mexican cheese with a moist, crumbly texture. An equal amount of shredded Monterey Jack cheese may be substituted for queso fresco.