Put the sweet potatoes in a bag and pour over 2 tbsp of the oil. Add a generous pinch of salt and rub them well to make sure they're coated. Tip onto a baking tray and slice a cross into the top of each one with a sharp knife. Put in the oven at 180 degrees Celsius for an hour. They're done when a knife cuts through them easily.
For the stew, finely chop the onion. Heat the remaining oil in a saucepan and add the onion, stirring, for two minutes.
Chop the mushrooms into chunks and add to the pan. Leave to cook until they are starting to turn golden.
Add the passata, lentils, smoked paprika and oregano. Cook gently for 5-10 minutes, then taste and season with salt and pepper.
Just before you are about to eat, add the chopped parsley to the stew. Slice the sweet potatoes and dollop the stew on top.
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Notes
If you wish, you can serve this with baked white potatoes rather than baked sweet potatoes. Prepare and cook them in the same way.
Leftover stew can be stored in the fridge for up to a week, or in the freezer for 3 months. Reheat thoroughly before serving.