Preheat the oven to 180°C. Put the sweet potatoes in a lightly greased baking tray and bake until you can easily push a skewer through them. Let them cool, then cut them half vertically. Scoop out most of the flesh, creating little “boats”, but leaving about 5mm flesh all around.
Meanwhile, finely chop the onion and garlic and pepper. Heat some oil in a saucepan and gently sauté the onion and pepper over low heat. When they are soft, add the garlic together with the spices and tomato purée. Stir for a couple of minutes.
Add the lentils and a cup of stock, then bring to a simmer. Turn down the heat and gently cook for 10-15 minutes or until you get a thick stew. Add more stock, if it’s too dry.
Mix in some chopped herbs and salt &pepper, then fill the sweet potato “boats”. Put these back in the baking tray, scatter some cheese on top (if used) and bake for 15 minutes or so.
Serve warm with a crispy salad.