Squash and tomato curry with lime and coconut

This curry is full of big, bold, clean, clear flavours. It is good with any type of flat bread and a tangle of blanched chard or spinach leaves dressed with lime juice and olive oil. Onion squash comes into season around this time of year: don't bother to peel off the skin – it is best left on.

1 medium onion squash
1tbsp vegetable oil
1 red onion, peeled and finely sliced
3 garlic cloves, peeled and finely chopped
1 green chilli, chopped (seeds left in)
10 curry leaves
A bunch of coriander, roots and stalks finely sliced, leaves reserved for garnish
1tsp mustard seeds
1tsp fennel seeds
2tbsp caster sugar, or to taste
2tbsp fish sauce, or to taste
Juice of 2 limes, or to taste
15-20 little ripe San Marzano tomatoes
340g/111/2oz jar (or tinned) good-quality peeled plum tomatoes
250ml/8fl oz coconut milk (fresh or tinned)

Using a large, sharp knife, slice through the middle of the squash. Scoop out the seeds using a spoon, then slice into 5cm wedges and set aside. Place a heavy-based pan over a medium heat. Add the oil and when it is warm, the onion. Lower the heat and cook for 10 minutes until soft. Add the garlic, chilli, curry leaves and coriander, and continue to cook gently.

Meanwhile, warm a small, non-stick frying pan over a medium heat, add the mustard and fennel seeds and cook until they begin to pop. Remove from the heat and pound to a powder, using a pestle and mortar. Add to the onion curry base, stir to combine and cook gently for a further five minutes. Add the onion squash, stir again and cook for 10 minutes, then add the sugar, fish sauce and lime juice. It is really important to get the balance of flavours right at this stage, so now is the time to taste and assess. The curry should be pleasantly (not aggressively) hot; sweet (but not sticky); sour (but not so much that it makes you squint); and salty enough to underpin and ground the dish.

Once you feel the flavours are right, add the little ripe tomatoes – squishing them slightly between your fingers as you do so, to help them release their flavour. Add the jar of plum tomatoes and cook for a further 20-25 minutes until the squash is tender when pierced with a fork. Pour in the coconut milk and cook for a final five minutes. The tomato and coconut milk enrich the curry, giving it a depth and smoothness that complete the dish.

Turn off the heat and allow to cool. Reheat the curry gently and thoroughly when you are ready to serve. Like many sweet dishes, this one improves in flavour if allowed to cool and sit before reheating.