Preheat oven to 350 degrees F. Butter a 13 by 9 inch shallow baking dish.
Spread hot mashed potatoes evenly across the bottom of the baking dish, coming up the sides somewhat to create a shell. In a large bowl, stir together the chicken, vegetables, soup, sour cream, onion or chives, mustard, garlic, and pepper.
Spread the chicken mixture on top of the mashed potatoes, and sprinkle evenly with the shredded cheese. Bake uncovered for 45 minutes (or bake for about 1 hour if made ahead and refrigerated), until cheese is melted and casserole is hot and bubbly.
Remove casserole from the oven, let stand for 10 minutes before serving. Good served with warm buttered biscuits.
