Place chicken breasts in a single layer across the bottom of a heavy based pan. Cover the chicken with the stock and bay leaves and bring the pan to the boil.
As soon as the water is boiled, reduce the heat so the water is barely simmering and partly cover the pan. Cook for 10 minutes.
Turn off the heat and allow the chicken to remain in hot water for 15 minutes before removing from the pan.
In a small pan heat the olive oil and add the flour and mustard powder. Using a small whisk, beat the mixture for 2 minutes then add 2 tablespoons of the stock used to cook the chicken. Continue to beat the sauce for a further minute or two.
Stir in the lactose free milk a little at a time to ensure a smooth sauce free of lumps. Season with a little salt and pepper.
When the milk is completely combined turn off the heat.
Cut the chicken into slices and arrange it in an ovenproof dish.
Cover the chicken with a layer of English spinach and pour the sauce over the top.
Sprinkle the Parmesan over the top of the sauce and place the dish in the oven preheated to 180 degrees Celsius for 5 - 7 minutes until the Parmesan is golden.