- Put the lentils and water in a small pan 
- Bring to the boil, stir and simmer until soft, stirring frequently - (Add more water if they get too dry but the mixture needs to be thick) - about 15 minutes 
- Add a little salt and leave to cool, then chill in the fridge 
- Meanwhile, fry the onions in the olive oil until they start to brown 
- Add the sugar and cook until caramalised 
- Preheat the oven to 220 degrees C 
- Stir the onions through the lentils 
- Add the lemon zest and juice 
- Add the rosemary and and some black pepper 
- The mixture can be chilled for up to 48 hours now if you are preparing in advance 
- Lay the pastry falt on a greased baking sheet 
- Spoon the lentil mixture into the middle in a fat sausage shape, leaving a border at each end 
- Wrap the pastry around the filling and press the edges together 
- Seal the ends of the Wellington 
- Brush the whole thing with a beaten egg 
- Score diagonally with a sharp knife several times 
- Bake for 20-25 minutes or until deep golden brown and puffed all over 
- Cool for a few minutes before slicing