Heat 2 tbsp of the oil in a large, non-stick sauté pan or shallow casserole dish. Add the onions and garlic and fry for 8 minutes over a medium to high heat until starting to turn golden, stirring occasionally. Pour the remaining tbsp of oil into the pan, tip in the carrots and butternut squash, and fry for 3 minutes.
Reduce the heat to medium. Stir in the lentils, chopped tomatoes, tomato puree, and stock. Season and simmer, covered, for 35 minutes over a low heat, until the carrots and butternut squash are tender and the lentils have softened. While this is cooking, skip to step 4 to get the potatoes on to cook.
Remove from the heat, stir in the kale, and stir while it wilts (approx. 2 minutes). Pour the mixture into a large baking dish (I used a square Pampered Chef stoneware dish – dimensions: 24cm x 24cm x 5cm deep). Leave to cool. Preheat the oven to 200C/fan 180C/400F/Gas 6.
Put the potatoes into a large pan and cover with cold water. Bring to the boil, add salt, and simmer for 15 minutes 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. Check the seasoning – I usually only need to add some pepper - then stir in the cheese
Spread the mash over the filling – use a fork or round-bladed knife to create an attractive, textured effect. Bake for 30 to 40 minutes until golden and bubbling. I turn it at 10/15 minute intervals to get it evenly browned.
Serve with an additional vegetable on the side if you like – boiled or steamed cabbage is good…and some homemade bread.