Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a large frying pan, add the onion and fry, stirring, for 5 minutes or until lightly browned. Mix in the garlic, tomatoes, stock, wine (if used), tomato purée and sugar and season with salt and pepper. Add the Puy lentils and bring to the boil.
Put the mushrooms in the slow cooker pot and pour over the lentil mixture, then cover and cook on low for 6–8 hours, stirring once if possible.
Serve with rocket leaves tossed with Parmesan shavings and a drizzle of olive oil and fried rounds of polenta.