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1
Preheat oven to 350 degrees F.
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2
Butter a 13 by 9 inch shallow baking dish.
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3
Spread hot mashed potatoes evenly across the bottom of the baking dish, coming up the sides somewhat to create a shell.
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4
In a large bowl, stir together the chicken, vegetables, soup, sour cream, onion or chives, mustard, garlic, and pepper.
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5
Spread the chicken mixture on top of the mashed potatoes, and sprinkle evenly with the shredded cheese.
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6
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.
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7
Remove casserole from the oven, let stand for 10 minutes before serving.
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8
Good served with warm buttered biscuits.
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9
Note: This casserole is even more over-the-top awesome made with a breadcrumb or stuffing topping. Assemble the casserole as directed, reserving the grated cheese. During the last 30 minutes of baking, top casserole with 1-2 cups freshly torn soft breadcrumbs (or stuffing mix), seasoned with a pinch of poultry seasoning and a few dashes of celery seeds. Drizzle with 2-3 tablespoons of melted butter, and then top with the grated cheese. Bake until topping is lightly browned and casserole is hot and bubbling. Extra yummy!