Measure the lentils into a saucepan and fill with enough water to cover them by 2 to 3cm. Bring to the boil and cook until tender, about 45 minutes. Check occasionally and add more water if needed. Drain and set aside to cool.
Preheat the oven to 190 C / Gas mark 5. Grease a 23x12cm (9x5 in) loaf tin.
In the bowl of a food processor, combine the onion, carrot, red pepper and wheatgerm. Pulse until finely chopped. Transfer to a bowl. Put the lentils into the food processor and process into a paste. Spoon the lentils into the bowl with the vegetables and mix in the rice, breadcrumbs, flax seed, egg whites, passata and olive oil. Season with thyme, cayenne pepper and salt. Spoon the mixture into the prepared loaf tin.
Bake for 45 minutes in the preheated oven, until heated through and browned on the top. Cool slightly before slicing and serving.