Spray a large 10x15” casserole dish with cooking spray.
Place chicken in the middle.
Prepare sauce on top of stove using medium saucepan.
Melt ½ stick butter (1/4 cup).
Add flour with whisk.
Whisk in half-and-half very slowly.
If sauce is still too thick, add a little milk.
Add chicken bouillon, mustard, dill weed, pepper and nutmeg.
Keep on low-medium heat and continue cooking until mixture thickens, a couple of minutes.
Spoon ½ to 1 cup of sauce over chicken in dish.
Add onions, zucchini and carrots to remaining sauce.
Stir to combine.
Spoon vegetables and sauce along sides of baking dish.
Melt remaining ½ stick butter (1/4 cup) in a small bowl; toss with Panko bread crumbs, cheese and walnuts.
Spoon over chicken and vegetables.
If desired, sprinkle entire casserole with a little more dill weed.
Bake, covered, about 30 minutes at 350°.
Remove foil and bake an additional 30 minutes or until chicken is done and carrots are cooked through.