Over medium-heat, saute the sliced garlic cloves in 1 tablespoon of butter until scent is released, add onion and celery and chicken thighs. Cook for approximately 10-15minutes. Until onion is soft and chicken has been THOROUGHLY browned.
Remove from frying pan and place into a large baking tray, leaving juices in the frying pan.
Melt another tablespoon of butter and the teaspoon of minced garlic in amongst the juices and add mushrooms. Cook for 5minutes, stirring constantly, and then add brocolli and cauliflower. Stir constantly and cook for another 5minutes. Then place in the baking tray. Juices and all.
Preheat oven to 180 degrees Celcius.
Mix together the flour, salt, and pepper. Melt the remainder of the butter and mix the flour mixture until well combined and smooth. Slowly top off with the chicken broth and mix well until smooth. Add the sour cream and tomato sauce. Mix well. Pour over what's in the baking tray.
Cover and bake for 45minutes, uncover and sprinkle with the grated cheese and bake uncovered for 10-15minutes or until cheese has browned.