Add the red lentils and vegetable stock to a saucepan, and bring to a simmer. Cook over a medium heat until the red lentils are tender and any excess liquid has cooked off - around 15 minutes. You'll need to stir regularly, especially in the last few minutes, to ensure the lentils don't stick to the pan.

Meanwhile, heat the oil in a frying pan, and add the chopped leek, carrot and green beans. Cook for a few minutes over a medium heat, until the leek is soft. Set aside to cool.

When the lentils are cooked, transfer them to a large mixing bowl, and add the cooked vegetables, rolled oats, smoked paprika, and a generous pinch of black pepper. Allow the mixture to cool for a few minutes, then add about 2/3 of the grated cheese (hold some back for topping). Mix thoroughly.

Spread the lentil mixture out into a greased baking tin (mine measured 9 inches square), and smooth out the top. Add the remaining grated cheese and a few slices of tomato. Bake at 180°C (Gas Mark 4 / 350°F) for around 45 minutes, or until the lentil slice is firm and the cheese on top is golden (cover with foil if you feel the cheese is browning too quickly). Either serve immediately, or allow to cool and cut into slices.