Put the vegetable oil in a large non stick frying pan set over a medium high heat. When hot, add the panch phoran and fry until the nigella seeds start to crackle and pop.
Turn the heat down to low, add the onion and dried red chillies and fry for 6-8 minutes until the onion turns golden brown.
Add the cubed potato, turmeric and chilli powder and fry gently, partially covered, for 20-25 minutes or until the potatoes are soft and cooked through.
Add the tomatoes and fry until the oil starts to separate then add the bay leaves and courgette. Stir together well and continue to cook for 10 minutes more or until the courgette is tender.
Finish with the salt and sugar and finally stir through the lemon juice just before serving.