Preheat the oven to 180°C/350°F/gas mark 4.
Heat the olive oil in a large pan. Add the chopped onion, celery and carrot and cook over a low heat for about 10-15 minutes until the onion is soft and translucent.
Add the crushed garlic and cook for a minute then stir in the flour.
Gradually stir in the wine (if using), followed by the stock. Mix in a little at a time to avoid lumps of flour.
Add the creme fraiche or cream, frozen peas and sweetcorn and some salt and pepper.
Bring up to a gentle simmer and cook, stirring often, for about 10 minutes until the vegetables are warmed through and the sauce has thickened. You can add a splash more stock as needed but the sauce should be fairly thick so don't add too much.
While the sauce is cooking cook the chicken strips according to the instructions on the packet. I prefer to gently pan fry them for the best flavour. Break up any large pieces into bite sized chunks then stir them into the sauce. Taste and add more salt and pepper as needed.
Meanwhile, peel and chop the potatoes, place in a large pan of cold salted water and bring up to the boil. Simmer until tender then drain well.
Mash the potatoes with the creme fraiche or cream, nutritional yeast and butter, adding a splash of milk as needed to create a soft, smooth mash. Season to taste with salt.
Pour the filling into an approx 20x25cm / 8x10 inch pie dish and spread level. Dollop the mashed potato on top and spread it out evenly then use the back of a spoon or the tines of a fork to rough up the surface a bit.
Bake the pie for about 25-30 minutes until golden and bubbling.
Leftovers will keep in the fridge for about two days.