Place the purple sprouting into a saucepan of salted boiling water and boil for 3-4 minutes. Remove from the water and place into a bowl of iced water to prevent further cooking. Drain the broccoli and season with salt and freshly ground black pepper.
For the sauce, place the cream, Brie, mustard and thyme into a small frying pan over a low heat and gently cook, until melted together.
Add the egg yolk and stir until thickened, then season well with salt and freshly ground black pepper.
To serve, place the purple sprouting broccoli onto a warm plate and pour over the cheese sauce.