Lentil Cheese and Tomato Pie

First make the filling: fry the onion in the butter until it is golden and beginning to soften, about 5-6 minutes, then add the split red lentils and garlic and stir for a moment or two before adding the stock or water. Bring the mixture to the boil, half cover and simmer for about 20 minutes until the lentils are soft and pale golden. Remove pan from heat and add tomatoes, cheese, basil, lemon juice, egg and seasonings. Leave mixture to cool Preheat the oven to 200C (400F) gas mark 6. Roll out two thirds of the pastry and use to line a 20cm (8in) flan dish or sandwich tin. Roll the remainder into a circle to fit the top of the dish. Spread the lentil mixture into the pastry case. Dampen the edges of the pastry with a little cold water then put the circle of pastry on top and press down lightly, crimping the edges. Prick the top and brush with a little beaten egg if you want a shiny finish. Bake the pie in the oven for about 40 minutes, until the pastry is golden brown and crisp.