Ain't nothing wrong with bringing more Dijon into your life. Here, you get to sauce a chicken breast with a luxurious mix of robust Dijon, white wine, and rich butter. But mustard isn't the only star of this dish: You'll never eat zucchini the same way after savoring it in this gratin, made with nutty Parmesan and jammy sun-dried tomatoes, and topped with crispy fried onions.
Trim zucchini ends and slice on an angle into ½" rounds. Cut sun-dried tomatoes into ¼" dice. Coarsely chop crispy fried onions. Pat chicken breasts dry, and season both sides with ½ tsp. salt and ¼ tsp. pepper.
Cook the Zucchini
Place a large non-stick pan over high heat. Add 2 tsp. olive oil and zucchini to hot pan. Stir often until browned but still crunchy, 2-3 minutes.
Make the Gratin
Remove pan from burner and stir in Parmesan, sun-dried tomatoes, ¼ tsp. salt, and a pinch of pepper. Transfer zucchini mixture to prepared casserole dishes. Place casserole dishes on prepared baking sheet to catch any drips. Wipe pan clean and reserve. In each dish, pour ¼ the cream over zucchini mixture. Bake until golden brown and bubbling, 10-15 minutes. Remove from oven, rest 5 minutes, then sprinkle crispy onions on top. While gratin bakes, cook chicken.
Cook the Chicken
Return pan used to cook zucchini to medium-high heat. Place 2 tsp. olive oil and chicken into hot pan. Cook until first side is browned, 3-5 minutes. Flip chicken, reduce heat to medium, and cook until chicken is browned and reaches a minimum internal temperature of 165 degrees, 4-6 minutes. Remove to a plate and rest 3 minutes. Reserve pan; no need to wipe clean.
Make the Sauce
Return pan used to cook chicken to medium-high heat. Add white wine, bring to a boil, and reduce to a light syrup, 2-3 minutes. Remove from burner, stir in Dijon, and swirl in butter. Sauce will thicken as it cools.
Finish the Dish
Slice chicken into ¼" slices, if desired. Place gratin on a plate and chicken in front of gratin. Pour sauce on top of chicken.