Melt butter in a large skillet. Add onions and cook over medium-high heat, stirring occasionally, for 20 - 25 minutes or until oninos are golden brown and caramelized.
Season chicken with lime juice and chile powder
Place equal amounts of onions, chicken, cheese, mango and cilantro on half of each tortilla.
Fold in half and cook in a large nonstick skillet over medium heat for 5 minutes on each side or until cheese is melted and tortillas are golden brown and crisp.
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Chicken, Caramalized Onion and Mango Quesidillas recipe