In small pan, saute the onion and lime juice with a splash of water for 2 to 3 minutes. Drain, rinse with cold water and drain again.
In a small bowl, combine the butter and oil. Set aside.
On one half of each tortilla, arrange 1/10 of the cheese, chilies and mango. Fold the tortillas in half. Brush both sides with the butter and oil mixture and place on a rimmed baking sheet. Bake for 5 to 10 minutes, until the cheese has melted and the tortillas are crispy. Cut each into thirds and serve hot.
Notes
To roast chiles, roast in the oven at a high heat (425 degrees F) until turning black in spots- or use tongs to hold chili over an open flame until it's roasted and black. Move warm roasted peppers to a zip bag, close and let steam for about 15 minutes. Open the bag and the skins should slide off of the chilies. Cut open & remove seeds too.