Saute the chicken and onions in the butter. Steam the broccoli and drain. Grease a square 8-inch casserole dish. Place the broccoli on the left side of the pan. Now, place the chicken down the middle. Place the peaches on the right side. Combine the mayonnaise and sour cream. Spread the mixture across the broccoli, chicken and peaches. Sprinkle with the Parmesan cheese and broil until brown bubbles appear.