Heat 1 tablespoon of the oil in a large saucepan over a moderate heat. Add the carrots and garlic and cook for 5 minutes, or until softened.
Stir in the mushrooms and lentils, then add the tomatoes, stock, cumin, ginger and sage. Bring to the boil, then reduce the heat to a simmer, cover and cook for 35 minutes, or until the lentils are tender. Add the peas to the stew for the last 5 minutes of the cooking time.
Meanwhile, heat the remaining oil in a large frying pan. Add the onion, sprinkle with the sugar and cook for 15–20 minutes over a moderate heat, stirring frequently, until the onion is lightly browned.
Season the stew to taste with salt and pepper, then serve hot with the onion heaped on top. Serve with rice or flatbread, if desired.
For an extra dash of flavour, crumble some cheese over the top to serve. Goat's cheese, feta or grated Gruyère would all be suitable.