Gently cook the onion in the butter for 10 mins without colouring. Meanwhile grade the zest from the orange, then peel it removing all the pith and chop the flesh. Add the garlic to the pan and stir to coat it in the butter, then add the orange pieces and the zest. Stir in the tomatoes, lentils and stock, bring to the boil, then reduce the heat and simmer for 20 minutes or until the lentils are cooked. Puree in a blender, then reheat gently and season to taste.
Serve garnished with chopped mint & orange zest