This is a recipe I came up with after trying several mustard sauces. I added walnuts to enhance the chicken flavor, and I added garlic to the mustard sauce for a bit of added flavor. I actually prefer this with more walnuts than is listed below. But the amount listed is what is preferred by most people for whom I've made this particular dish. I hope you like it.
Mix together 1/2 cup of the chicken broth. 1/4 cup of the chopped walnuts, and 2 teaspoons minced garlic. Pour it in an 8x8 or 9x9 baking dish.
Arrange chicken pieces in the baking pan, turning to be sure all pieces are well coated with the nutty broth sauce. Season both sides of the chicken breasts with basil, salt and pepper. Place the pan in the refrigerator to marinate for about 1 hour. (If you're running short on time, it's OK to skip the 1-hour refrigeration step; just be sure the chicken is well coated on all sides with the nutty broth sauce.).
Bake chicken breasts in the nutty broth sauce for about at 375°F for 30 to 45 minutes, or until juice runs clear in the chicken.
When chicken is about half done (about 15-20 minutes into the roasting time), sauté the remaining 1/4 cup walnuts and remaining 1/2 cup broth in a skillet over medium heat for about 3-4 minutes. Add the sliced mushrooms, and stir, continuing to sauté until the mushrooms are tender.
In separate skillet, sauté the remaining 2 teaspoons garlic in the butter for about 2 minutes over medium-low heat. Add the Dijon mustard and stir well. Add the cream and continue cooking, stirring constantly, until the sauce just begins to thicken.
Pour the mustard sauce over the chicken breasts. Top with the nutty mushroom mixture. Sprinkle chopped green onions (scallions) over the top, if desired.