Heat the oil in a frying pan and fry the red onions over a medium heat for about 8 minutes until soft and golden. Add the garlic and cumin seeds and cook for a further 5 minutes.
Mix the onion mixture into the lentils, season well and leave to cool.
Make the dressing by mixing together the ingredients in a small bowl.
Serve the cooled lentils on a bed of leaves, with the grated beetroot and apple (if used), a couple of spoonfuls of minty yogurt and a generous squeeze of lemon juice.