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. 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. Pickled onions may be prepared ahead and refrigerated as long as overnight, if desired. 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. 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. 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.