Heat a deep fat fryer to 180°C.
Toast cumin and mustard seeds in a hot frying pan for 20 seconds until they pop.
Add onion, 15ml of oil, garlic, ginger, and chilli, and cook until softened.
Mix the cooked onion mixture with the cold mashed potato in a bowl.
Stir in coriander and season with salt and black pepper.
Form the mixture into cylinders 6cm long and 2.5cm wide.
Dust each cylinder with flour and dip in the beaten egg.
Coat the potato in a mixture of sesame seeds, black onion seeds, and breadcrumbs.
Fry the croquettes for 3 to 4 minutes until golden-brown and drain on kitchen paper.
Blend coriander, mint, garlic, and 15ml of oil into a fine purée.
Add yoghurt to the blender and process until smooth.
Season the chutney with lime juice, salt, and pepper.
Serve three croquettes per plate with a spoonful of chutney on the side.