01 -
Preheat the oven to 232°C (450°F).
02 -
Pat chicken dry and season with salt and pepper. Place chicken in a medium saucepan with 3 cups of chicken broth. Bring to a gentle boil, cover partially, and simmer for 15 minutes or until cooked through. Remove chicken, shred with forks, and reserve the cooking broth.
03 -
Melt butter in a large (30 cm) skillet over medium heat. Add onions and sauté for 4 minutes. Add garlic, onion powder, mustard powder, dried thyme, dried rosemary, ground sage, and ground black pepper. Cook for 1 minute more, stirring well.
04 -
Add flour to the pan, stir to evenly coat, and cook for 2 minutes to eliminate the raw flour taste.
05 -
Reduce heat to low. Gradually add 2 cups of reserved chicken broth, stirring well after each addition. Slowly add half-and-half in small increments, continuing to stir. Stir in chicken bouillon and Worcestershire sauce.
06 -
Add shredded chicken to the skillet, mixing thoroughly. Increase heat to medium and gradually add the remaining chicken broth, a little at a time, while letting the filling simmer and reduce for 10–15 minutes.
07 -
Stir in frozen mixed vegetables and heat through. Remove skillet from heat.
08 -
Transfer filling to a lightly greased 23 x 33 cm (9 x 13 inch) baking dish if the pan is not oven-safe. Let the filling cool slightly. Using a silicone spatula, gently scoop and evenly spread the warm mashed potatoes over the filling, being careful not to compress.
09 -
Bake uncovered for 10 minutes in the preheated oven.
10 -
Set broiler to high (288°C/550°F) and broil for about 4 minutes, monitoring closely to achieve a lightly browned top without burning. Do not broil in glass or Pyrex baking dishes.
11 -
Remove from oven, garnish with chopped parsley if desired, and allow to rest for 5 minutes before serving.