Put the lentils, tomato purée and stock in a saucepan and bring to the boil. Reduce the heat, cover tightly and simmer for 20 minutes. Add the bulgar wheat and lemon juice and season to taste with salt and pepper. Cook for 10 minutes until all the stock has been absorbed.
Meanwhile, heat the oil in a frying pan, add the onions and sugar and cook over a low heat until deep brown and caramelized.
Stir the mint into the lentil and bulgar wheat mixture, then serve warm, topped with the fried onions and chopped tomato.