Hollow out 16 large mushrooms (I used the stems from these mushrooms to get my 100g chopped mushrooms, to avoid waste - save the leftovers for another dish!) and set aside. Place mushroom pieces, celery and onion into food processor and pulse until finely diced - place into a seperate bowl. Place chicken into food processor and pulse until finely diced - mix with veggies.
Heat chicken broth and water in pot, dump in chicken-veggie mixture with seasonings, and simmer until all liquid has reduced and veggies are tender. Crush Melba toasts with a rolling pin to make breadcrubs and toss them in the pot.
Place hallowed out mushrooms into a baking dish and spoon filling in, making sure to pack tightly.