Pre-heat the oven to 200C. Split the leeks in half, lengthways, keeping them intact at the base. Run them under a tap and rinse out all the mud between the layers.

Slice the dark green part of the leeks into 1-2cm chunks and the mushrooms into halves or quarters and sauté with the butter or oil and a pinch of salt.

When the mushrooms and dark greens have started to cook down a little, slice the pale part of the leeks and add that to the pan too with an extra pinch of salt.

Peel and slice the garlic cloves and pull off the thyme leaves and add them to the pan too. Sauté gently for around 10 minutes or until very fragrant and the vegetables are softened and mostly cooked through.

Add the mustard and either a half glass of white wine or a tiny splash of vinegar and stir well.

Sprinkle in the flour and nutritional yeast, stir to coat the vegetables, and then add generous splashes of oat milk and keep stirring and cooking out the flour until you get to to a silky, creamy consistency.

Taste and adjust the seasoning with pepper and more salt as needed. Then drain the butterbeans (reserve the aquafaba for mayonnaise or clafoutis) and add them to the pie filling.

Then either tip the filling into a pie dish and cover quickly with pastry or pop the pastry straight onto your pan if it is oven and hob safe. Work to get the pie in the oven quickly if your filling is still hot otherwise the pastry will melt.

Put the pie into the oven for about 25 minutes or until the pastry is cooked through and the filling is bubbling. Slice into wedges and serve with steamed greens and some extra wholegrain mustard on the side.