Meanwhile crack on with the filling. Heat 1 tbsp oil in a large saucepan. Bung in the onions and thyme, cook for 5 mins.
Add in the garlic and tomato puree, cook for 1-2 mins.
Tip in the rest of the ingredients, apart from arrowroot, salt and pepper.
Simmer on high for 10-15 mins, until the veg and lentil are soft and the liquid has reduced by about 1/2. (You dont want it too saucey for filling the potatoes)
Mix the arrowroot with 1 tbsp of water. Wth the saucepan still on the heat, pour in the arrowroot little by little, stirring constantly until you have a thick sauce that coats the lentils and veg, adding additional arrowroot if needed.
Take off the heat and season with salt and pepper to taste.