Meanwhile, heat the remaining oil in a frying pan. Add the onion and mushroom stems and cook for 3 minutes until beginning to soften. Add the garlic and spinach and season with salt and pepper.
Peel the potatoes and cut into ½ cm slices. Place in a pan of cold water and simmer until just cooked. Drain well. Melt the butter in a pan and add the flour and mix to a smooth paste.
Aim to get the filling cooked the day before if time allows. Then all that is left is to fill the pies, cook and enjoy. This can be made as one big pie if you don't have individual tins.