Preheat the oven to 400°.
Heat the butter in a large frying pan over a medium heat. Add the leeks and cook stirring for 3-4 minutes, or until softened.
Add the celery and bay leaf and increase the heat to high, cook for 1-2 minutes.
Add the chicken broth to the pan and bring to the boil.
Add the carrots, fennel and mushrooms and reduce the heat until the mixture is simmering. Continue to simmer the mixture until the volume of liquid has reduced and the vegetables are tender, about 12-15 minutes.
Meanwhile, for the topping, fill the bottom of a pot with 1/2” of water and add the cauliflower, and garlic cloves.
Bring the pan contents to a simmer over a medium heat, cover and cook for 6-8 minutes, or until the cauliflower is tender when pierced with a fork (add a splash more water during cooking if necessary).
Remove from the heat and transfer the contents to a food processor. Blend till smooth and creamy {similar to the texture of mashed potatoes}. Season with cheddar cheese, salt and pepper. Pulse food processor to blend.
To finish the pie filling, remove the bay leaf and add the shredded chicken, parsley, salt and pepper. Stir to blend and pour into a pie dish.
Spoon the cauliflower topping over the filling and spread into an even layer using the back of a spoon to smooth.
Dot the top of the pie with the 1 tablespoon of diced unsalted butter.
Bake for 25-30 minutes, or until the filling is bubbling and the topping is golden-brown.