Trim fat from roast. Add onion mixture. Transfer to serving platter. Serve with picante sauce mixture.
Halve peppers lengthwise and core.
Place cut side down on rack and broil 7 minutes or until lightly charred. Meanwhile, in a medium bowl, mix together all other ingredients. Turn peppers over and stuff peppers with corn mixture. Broil 3 to 5 minutes or until filling is hot. Marinate meat, turning occasionally, for several hours at room temperature or overnight in refrigerator.