Melt the butter in a large saucepan
Add the chopped onion and fry gently until soft
Add in the flour and cook on a low heat for a few minutes stirring all the time
Switch to a whisk and gradually add the milk a little at a time whisking constantly. As the milk is absorbed into the flour add more but do not add it all at once or your sauce will be lumpy.
Add the crushed stock cube, dried herbs and seasoning
Cook for 5 minutes over a low heat. The sauce should be thick but not solid, add more milk if seems very thick.
Add in the nutritional yeast and cooked vegetables
Pour into greased pie dish and leave for 20 minutes to cool
Preheat oven to 200°C/400°F/Gas Mark 6
Cut a 2cm strip of pastry and put around the outer top edge of the pie dish
Top with the sheet of pastry and press edges to seal. Use any trimmings to decorate
Brush with vegan milk and poke a few holes to let steam escape
Cook for 25 - 30 minutes untiil the pastry is golden brown
Let sit for 10 minutes to cool a bit before serving