Mustard chicken is one of those recipes that has many, many variations. You could start with the classic in "Mastering the Art of French Cooking" or the almost equally traditional (from Old New York) "Delmonico's Deviled Chicken" in "The Fanny Farmer Cookbook" (eleventh edition). The New York Times Cooking section has at least a dozen choices and I am sure that almost any good cookbook or website you opened would offer a few more.
I am delighted, however, to report that my long standing quick and easy version is almost identical to my hero Mark Bittman's in his "Simple Chicken Recipes" collection.
Be warned: this simple dish is not a thing of beauty, but it is very good indeed - especially cold the next day for lunch.
ALL YOU NEED is a chicken breast per person (or legs/thighs if you prefer), your mustard of choice, plus chopped onions/spring onions (optional but nice)
Heat oven to 400F. Place chicken pieces in oiled baking dish. Mix chopped onion and mustard - enough to spread over the top of each piece of chicken - using any extra to baste the chicken if needed. Roast for 15-20 minutes or until chicken is cooked through.
Note: if the chicken breasts are very large, I slice them in half horizontally so they cook more quickly.
Some sliced potatoes sautéed in olive oil are very nice with this.