Cook the broccoli florets in a pot of boiling salted water for 4 minutes and then shock them in cold water, drain and set aside.
Pan fry your chicken fillets in a small amount of butter with some salt and pepper until the chicken has a nice golden brown crust, then roast for 20 mins at 180deg C. to cook through.
For the sauce prepare your white sauce, but add leeks and chicken stock at the same time as the garlic and onion.
Slice the chicken and arrange in an oven tray, add the broccoli and pour over the sauce, then top with the cheese and breadcrumbs.
Bake at 180deg C . for 30 minutes until it is golden brown and crispy on top.