Place chopped onion on the bottom of a 12-cup or larger Crock Pot. Add chicken thighs, beans and corn. In a small bowl, mix together 1 cup salsa, chiles, garlic and cumin. Pour over top. Cover and cook on low for 8-10 hours.
Remove chicken pieces to a plate and, using two forks, pull the meat into bite-sized pieces. Return meat to pot. Stir in cilantro.
Peel potatoes, quarter, place in a medium pan, add salt, cover with water and heat to boiling. Reduce heat and cook potatoes until fork-soft, about 10 minutes. Drain the potatoes and keep in pan.
At this point, move one oven rack to 6 inches from broiler, and the other to the bottom rung. Turn broiler to high. Place a 3-quart casserole dish on the bottom rack to heat up while you prepare the potatoes.
To the drained potatoes in the pot, add sour cream and remaining 1/2 cup salsa. Using a measuring cup or soup ladle, scoop 1/2 cup of liquid from the chili, add to potatoes and mash quickly with an electric beater. Continue to add chili liquid to the potatoes a little at a time until whipped creamy. Stir in shredded cheese.
Remove casserole dish from oven, scoop the chili into the hot dish, top with mashed potatoes and use a fork to spread the potatoes to seal to the edge, then place under the broiler for at least 5 minutes, watching carefully to not over-brown. Serve immediately.