Heat olive oil in a large frypan over medium heat. Cook onions until deep golden brown (about 15 minutes). Push onions to one side of the pan, then add mushrooms. Once mushrooms have started to brown, season with salt, then stir to combine with the onions. Add garlic, rosemary, sage, cayenne pepper and a good grind of black pepper, and cook for another few minutes.
Add balsamic vinegar to the pan and stir until the liquid has mostly evaporated. Add lentils, tomato paste and marmite, and stir to combine. Sprinkle 1 tablespoon of flour, and stir to combine. Cook for a few minutes, stirring often.
Add the milk to the pan and stir to combine. Cook for another few minutes, stirring, until the mixture is dark, glossy and thick. The final consistency should be like a thick gravy. If the mixture is too thick, add some more milk.
Shortcrust Pastry
Preheat oven to 180°c Fan (390°F). Add the flour to a bowl and rub in the butter until like fine breadcrumbs. Add the cold water and mix in until it forms a dough. Roll out onto a floured surface to about 1cm thick. Get the serving dish or tin and place it upside down on the pastry. Cut around the dish with a knife to give you the right size to cover the pie.
Add the filling to the tin and cover with the shortcrust pastry. Pierce two holes in the middle and decorate with any leftover pastry. Brush with milk or a beaten egg and bake in the oven for 30 minutes.
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