To make the pie filling and creamy sauce
In a large saucepan, add the olive oil and warm over a medium heat. Once heated, add the onion and a large pinch of salt. Sweat down for 3-5 minutes until it has started to soften but don’t let it catch or burn.
Add the veggie bacon and continue to cook for 3-5 minutes to crispen. Next add the celery and cook for a further 2 minutes.
Deglaze with the white wine, stirring well to burn off the alcohol. Once there is no liquid left in the bottom of the saucepan, add the leeks, followed by the cauliflower. Cook the vegetables down together for around 8 minutes, until everything is starting to soften. Then add the herbs; rosemary, sage and thyme, followed by the nutritional yeast.
In a small bowl mix together the plain flour and oat milk until smooth, before adding the rest of the ingredients for the creamy sauce. The mixture should be a relatively thick and smooth consistency. Season to taste.
Add the sauce to the pie filling mixture, check the seasoning is to your liking and set aside to cool slightly.
Brush a cake tin or pie dish with olive oil, before layering in the sheets of filo pastry one by one. Use a pastry brush to lightly coat each filo sheet thoroughly with olive oil before adding the next. Overlap the sheets so that they form a layered base in your dish, about 6-8 sheets in total. Add the cooled pie filling mixture and top with vegan cheese, if using.
Layer the top of the pie with 3-5 more filo sheets, and fold in the edges to form a rough looking crust. Brush thoroughly with olive oil and sprinkle with nigella seeds.
Bake for 20 minutes in the preheated oven until the pie is golden brown all over.
Serve with your favourite festive sides – roast or mash potatoes, spring greens or steamed broccoli and roasted carrots.