For the croquettes, cook the Puy lentils according to packet instructions. Drain.
Mix the Puy lentils, sweet potato, mixed herbs, breadcrumbs, nutmeg and egg yolk together in a bowl until well combined.
For the coating, sprinkle the flour onto a plate, beat the eggs in a bowl and sprinkle the polenta onto a separate plate.
With floured hands, mould small amounts of the mixture into 7.5cm/3in long cylinders.
Dredge the croquettes in the flour, then dip into the beaten egg and finally dip into the polenta to coat completely.
Heat the oil in a deep heavy-based frying pan until a breadcrumb sizzles and turns brown when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Fry the croquettes for 2-3 minutes, or until crisp and golden-brown. Remove from the pan with a slotted spoon and set aside to drain on kitchen paper.
For the raita, mix the yoghurt, mint and lime zest and juice together in a bowl.
Serve the croquettes with the raita dip alongside.