Cut the heads of cauliflower into florets. Don't discard the stems, they are delicious when roasted. Place all the cauliflower into a roasting dish then drizzle over the olive oil and season with salt.
Place in the oven and allow to roast for 30 minutes or until the cauliflower is golden brown and tender.
Make the cheese sauce according to recipe instructions.
Combine the cauliflower with the cheese sauce in a suitable oven-safe dish. Top with a sheet of puff pastry then brush with beaten egg. With a sharp knife, make a hole to allow steam to escape.
Place the pot pie in the oven and allow to bake for 15-20 minutes until the pastry is golden brown and crisp.
Remove from the oven and allow to cool for 5 minutes then serve.