Heat 2 tablespoons of the oil in a large, non-stick saute pan. Add the onions and garlic and fry for 8–10 minutes over a medium–high heat or until starting to turn golden, stirring occasionally. Pour the remaining tablespoon of oil into the pan, tip in the carrots and swede, and fry for 3 minutes.
Reduce the heat to medium. Stir in the lentils, tomatoes, sun-dried tomato paste, and stock. Season and simmer, covered, for 35–40 minutes over a low heat, until the carrots and swede are tender and the lentils have softened.
Remove from the heat, stir in the spinach, and let it wilt. Check the sauce’s consistency. Pour the mixture into the baking dish. Leave to cool. Preheat the oven to 200C.
Put the potatoes into a large pan and cover with cold water. Bring to the boil, add salt, and simmer for 15–20 minutes or until tender. Drain well. Put the butter and milk in the pan and warm through over a low heat. Remove from the heat, return the potatoes to the pan, and mash with the milk and butter, then whisk until smooth. Season, then stir in the cheese.
Spread the mash over the filling. Bake for 30–40 minutes until golden and bubbling.
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