Finely chop the onions. Cut the tomato and cucumber into 1cm chunks. Thinly slice the chilli, removing the seeds if less heat is desired.
Step 2 — Caramelise onion
Heat 1 tbs oil in a medium saucepan over medium-high heat. Cook three-quarters of the onion, stirring regularly, for 5-7 mins until starting to caramelise.
Step 3 — Make kachumber
Meanwhile, combine the tomato, cucumber, remaining onion, ½ tsp sugar and 1 tsp white vinegar in a large bowl and season with salt and pepper.
Step 4 — Start curry
Add the masala seasoning to the onion and cook, stirring, for 1 min or until fragrant. Stir in the mango chutney, three-quarters of the coconut milk and 125ml (½ cup) water and bring to a simmer. Reduce the heat to medium and cook, stirring occasionally, for 5 mins to develop the flavour. (It's important the curry doesn't boil or the coconut milk may separate.)
Step 5 — Warm tortillas
While the curry is cooking, heat a chargrill pan over medium-high heat until hot. Warm the tortillas, in batches, for 30 secs each side or until heated through and slightly charred. Cover with a clean tea towel to keep warm. (To microwave, just before serving, put the tortillas on a plate, slightly overlapping, and microwave for 30 secs or until warmed through.)
Step 6 — Finish curry and serve
Add the prawns to the curry and cook, stirring occasionally, for 2-3 mins until just cooked through. Remove the pan from the heat. Taste, then season with salt and pepper. Divide the prawn curry among bowls, then drizzle over the remaining coconut milk and scatter with the chilli, if desired. Serve with the kachumber and tortillas.