Finely chop the onion, celery, garlic, carrot and sweet potatoes.
Heat the olive oil in a pan and gently cook the onion and celery until softened. Add the garlic, carrot and sweet potatoes, cover, and cook over low heat for about 8 minutes.
Stir in the oregano, smoked paprika, cumin and tomato purée, mixing well to coat everything.
Add the chopped tomatoes and gravy or stock. Bring to a boil, then reduce the heat and simmer for 10 minutes.
Stir in the lentils and greens, and cook for another 5 minutes until the vegetables are tender. Add a splash more stock if the mixture feels too thick.
Cut the leftover lamb into smaller pieces if needed, then stir it into the mixture along with the parsley. Turn off the heat.
Transfer the filling to a deep baking dish and leave to cool slightly.
Preheat the oven to 180°C.
Roll out the pastry and cut it so it’s about 2 cm larger than your dish. Lay it over the top and press the edges to seal.
Use any leftover pastry to make simple decorations if you like. Stick them on with a little beaten egg.
Cut a couple of small slits in the pastry to let steam escape, then brush with egg for a golden finish (optional).
Bake for about 20 minutes, or until the pastry is golden and crisp.
Serve warm, with a fresh green salad on the side.