Saute onion, leek, carrot and garlic in a tablespoon of olive oil until soft and starting to caramelize.
Add the broccoli, peas and water. Cover with a lid and allow to steam for 5-7 minutes until the broccoli is just cooked. Test by piercing the stem with a sharp knife. If there is a lot of liquid in the pan, drain before combining with the sauce.
Prepare the bechamel/cheese sauce as directed. Pour cheese sauce over the cooked vegetable and mix.
Transfer the filling to a baking dish.
Cover with thawed puff pastry. Crimp the edges then cut a few vents into the surface. Brush with egg wash.
Place the pot pie in the oven and allow to bake for 20-30 minutes until the pastry is golden brown and puffed.
Remove and allow to rest fof 5-10 minutes before serving.