Put the lentils in a heavy saucepan and pour in enough water to cover them by 7 1/2 cm. Bring to the boil, then reduce the heat and simmer for 20–30 minutes or until tender. Drain well and keep warm.
While the lentils are cooking, heat 1 1/2 teaspoons of the oil in a large non-stick frying pan over a high heat. Add the onion and stir to coat with the oil, then reduce the heat and cook, stirring frequently, for about 20 minutes or until soft. Stir in the sugar, increase the heat and continue cooking, stirring, until the onion is dark brown and crisp. Immediately pour onto paper towels to drain.
To make the tomato sauce, heat the oil in the saucepan you used to cook the lentils. Add the garlic and sauté for 30 seconds. Stir in the turmeric and sauté for a further 30 seconds. Pour in the tomatoes and add the sugar. Bring to the boil, stirring. Reduce the heat and simmer, stirring occasionally, for 10–15 minutes or until the sauce thickens a little. Season with pepper.
Heat the remaining oil in the frying pan, add the cumin, ground coriander and cayenne pepper, and fry, stirring, for 30 seconds. Add the lentils and pepper to taste. Keep warm over a very low heat.
Cook the macaroni in boiling water according to the packet instructions. Drain well. Spoon the macaroni onto a serving platter and top with the spiced lentils and the tomato sauce. Sprinkle with the chopped fresh coriander and top with the crisp onions. Serve at once.