Method

1. Put the lentils in a small pan and add the water. Bring to the boil and cook, stirring frequently, until soft - about 15 minutes. You may have to add a little more water while cooking if they seem to be too dry, but don't add too much as the lentils should be thick. The resulting consistency should be like thick porridge. Stir some salt through the lentils cool. Chill in the fridge to firm up the lentils.

2. Heat a frying pan over a moderate heat and add the oil. Fry the onions, stirring frequently, until they start to turn golden. Add the sugar and cook until well caramelised. Preheat the oven to 220C/gas 7.

3. Stir the onions through the lentils, adding the lemon zest and juice, rosemary and black pepper. Taste for seasoning. The mixture can be chilled and stored for up to 48 hours at this stage, until you're ready to cook the Wellington.

4. Lay the pastry flat on a baking sheet. Spoon the lentil mixture down the middle in a thick sausage shape, leaving a border at each end. Wrap the pastry around the filling and press the edges firmly together. Seal the ends of the Wellington. Roll the pastry over so the seam is underneath. Brush with beaten egg, then slash with a sharp knife several times diagonally.

5. Bake for 20-25 minutes, or until deep golden and puffed all over. Cool for a few minutes before slicing.