1. Preheat broiler. Place cheese in large shallow, oiled earthenware casserole or cast iron skillet. Slice chiles and peppers lengthwise and scatter over the cheese. Place under broiler just until cheese begins to melt. Scatter onions over peppers and continue to broil just until onions begin to color and cheese begins to bubble.
2. Wrap tortillas in damp paper towels and heat in microwave on med-hi (75%) power about 15 seconds or until warmed. To serve, scoop melted cheese into tortillas.