Heat oven to 350 degrees F.
Combine soup and mustard in large bowl. Add chicken, artichoke hearts, peas and onion; mix well. Spoon chicken mixture into 11 x 7 x 2-inch baking dish.
In same bowl, toss together frozen mashed potatoes and mayonnaise until well mixed. Spoon evenly over chicken mixture.
Bake in 350 degree F oven for 30 minutes or until bubbly.
Increase oven temperature to broil. Broil casserole until top is golden, 8 minutes. Let stand 15 minutes. Makes 6 servings.