Preheat oven to 450°F. Pat chicken dry, season with salt and pepper. Add chicken broth to a medium saucepan with the chicken. Bring to a gentle boil, then reduce heat to maintain a low simmer with lid cracked. Cook 15 minutes until chicken reaches 165°F. Remove chicken, shred with forks, and reserve the cooking broth. While chicken simmers, prepare your mashed potatoes if making them fresh.

While chicken cooks, melt butter in a 12-inch oven-safe skillet over medium heat. Add onions and sauté 4 minutes until translucent. Add garlic and all dried seasonings, cooking 1 minute while stirring constantly.

Sprinkle flour over the onion mixture and stir to coat. Cook 2 minutes, stirring frequently, until raw flour smell disappears and mixture looks pale golden and smells nutty.

Reduce heat to low. Slowly add 2 cups reserved chicken broth in small splashes, stirring between each addition. Add half and half the same way to maintain a smooth consistency. If lumps form, whisk vigorously to smooth them out.

Stir in bouillon and Worcestershire sauce, then add shredded chicken. Increase heat to medium and gradually add about 3/4 cup more broth while simmering for 10-15 minutes until thickened. The gravy should coat the back of a spoon. Add frozen vegetables during last 2 minutes and stir until heated through.

If skillet isn't oven-safe, transfer filling to greased 9x13-inch baking dish. Let cool 5 minutes. Drop large scoops of warm mashed potatoes across the top, positioning them close together. If using leftover mashed potatoes, warm them in microwave with a splash of milk first. Gently spread with silicone spatula using light strokes. Create ridges with fork if desired.

Bake uncovered 10 minutes. Switch oven to high broil at 550°F and broil 4 minutes, watching closely, until peaks turn golden brown. Don't use glass or Pyrex baking dishes as they can shatter under high heat. Let rest 5 minutes before serving.