Place the chicken into a large pot and cover with water. Add the bay leaf and black pepper. Bring to the boil over a medium-high heat, then reduce the heat and leave to simmer for 1 hour. Lift out the chicken and leave to cool. Let the stock boil until approximately 1 cup (250 ml) remains. Remove the bay leaf and pepper and set the pot to one aside. When cool enough to handle, pick the meat off the chicken.
Meanwhile, in a medium-sized pot, melt the butter over a medium heat. Stir in the flour followed by the milk and the reduced stock. Bring to the boil and leave to simmer until the sauce thickens enough to coat the back of a spoon.
Return the large pot to the heat and add the butter. When the butter is completely melted, add the onion and leave to simmer for 1 minute. Add the sauce, then add the celeriac, the carrots, the cauliflower and the leek and leave to simmer for 10-12 minutes, or until the vegetables are tender. Mix in the meat and add salt and more spice to taste.