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TOSS lettuce with radishes, onions, vinegar, salt and pepper; set aside.
REMOVE bread from insides of bolillo halves, leaving 1/4-inch-thick shells; reserve removed bread for snacking or another use. Spread insides of shells evenly with mayo. Spoon beans into bottom halves of bolillos; top with Singles. Place on rack of broiler pan.
BROIL 2 min. or until Singles are melted. Top with lettuce mixture; cover with tops of bolillos.