For the croquettes, heat a deep fat fryer to 180C/350F (CAUTION: Do not leave hot oil unattended).
Heat a frying pan until hot, add the cumin and mustard seeds and cook for 20 seconds until they start to pop (take care to avoid the seeds popping into your eyes and face).
Add the onion, one tablespoon of vegetable oil, the garlic, ginger and chilli and cook for a couple of minutes until just softened.
Tip the cooked onion mixture into a bowl with the mashed potato and mix well.
Add the coriander leaves and season with salt and black pepper, then mix once more.
Taking spoonfuls of the mix, form little cylinders about 6cm/2½in long, and 2.5cm/1in wide.
Prepare a tray of the flour and a bowl of the egg. Dust the croquettes with flour and then dip in the egg, coating on each side.
Mix the sesame and black onion seeds with the breadcrumbs on a plate, then coat the potato in the crumbs, ensuring all sides are covered.
Place in the fat fryer and cook for 3-4 minutes, or until hot through, and crispy and golden-brown outside. Drain on a plate lined with kitchen paper.
For the chutney, put the coriander, mint and garlic into a blender with the vegetable oil and blend to a fine purée. Add the yoghurt and blend once more until the herbs are very fine.
Season with the lime, salt and a little black pepper.
To serve, place three croquettes onto each plate and spoon the chutney alongside.