Heat the vegetable oil in a small saute pan over medium-high heat. Lightly fry the jalapeno chiles, turning them in the pan until their skins are blistered, about 5 minutes. Set aside to cool. Once cooled, stem, seed and finely chop the jalapeno chiles and place them into a large mixing bowl. Add the cilantro, mangoes, lime juice and onions and season with salt and pepper. Serve immediately or store the salsa in the refrigerator in an airtight container for up to 3 days.