Once hot, add the leek, fennel and carrot, and fry for about 10 minutes until softened
Add the white wine and fry for a couple of minutes until all the wine has evaporated
Measure out the milk in a jug or other container, add the cornflour to the container and stir until well mixed
Pour the milk and cornflour mixture into the pan, and add the fresh parsley or dill, capers, white beans, kale or spinach, and salt + pepper and mix well
Bring to the boil and stir regularly, until thickened
For the mashed potatoes:
Place potatoes in a pan along with a pinch of salt, and roughly cover with water
Bring to the boil and simmer on a low heat for around 15 minutes, until soft enough to gently pierce with a fork
Drain away the water and keep the potatoes in the pan
Add the parsley or dill, watercress, nutritional yeast, and salt + pepper and use a potato masher (or a fork and a lot of elbow grease) to mash the potatoes until smooth
Preheat the oven to 180 degrees Celsius (350 degrees Fahrenheit)
Transfer the sauce into a large baking dish and top with the mashed potatoes
Score the top of the mashed potatoes with a fork to roughen the surface for more crispy bits!
Bake in the oven for 30-40 minutes, until the top is golden brown and the sauce is bubbling nicely