Debone your chicken thighs and remove the skin or use skinless, pre-deboned chicken thighs. Place your skinless, deboned chicken thighs in a 9”x13” pan lined with parchment paper. Season with salt and pepper to taste. Cover the pan with aluminium foil and bake at 400℉/200℃/Gas Mark 6 for 45 minutes. Remove from the oven and shred with two forks going in a crisscross direction. Set aside.
While your chicken thighs are baking, wash your broccoli and cut the broccoli heads apart into bite site size florets. Place into a medium bowl. Pour just a little bit of olive oil onto the florets and toss around. Season with salt and pepper. Place into a 9”x13” pan lined with parchment paper and bake for 30 minutes at 400℉/200℃/Gas Mark 6. Once baked, add to the bowl with the shredded chicken thighs.
Wash and slice the celery stalks. Add to the bowl.
In a small bowl, add your breadcrumbs. Season them to taste. (I like to add sage, rosemary, thyme, salt, and pepper.) If you do not want to season the breadcrumbs, add directly to the large bowl with the other ingredients.
In a separate medium bowl, add the can of cream of mushroom soup and cup of milk. Whisk or beat on medium speed until incorporated together. Pour into the bowl with all of the ingredients.
Add 1 c. of shredded mozzarella cheese. Mix all of the ingredients together.
Pour into a greased 9”x13” pan and bake uncovered for 20 minutes 400℉/200℃/Gas Mark 6. Remove and pour the last 1/2 c. of mozzarella cheese on top and return to oven for 5 minutes to let the cheese melt.
Remove from oven, let cool, and serve.