Peel the onion and finely slice along with the chillies. Peel and finely chop the garlic and ginger. Discard the outer layer of the lemongrass and finely chop.
Heat 1 to 2 teaspoons of oil in a pan and sauté the onion, garlic, chilli, ginger, lemongrass and lime leaves (if using) over a medium heat for 4 to 5 minutes, until soft but not coloured.
Add the prawns, vegetables, coconut cream and stock (or 100ml of water) to the pan. Bring to a gentle simmer and cook for 3 to 5 minutes, or until the prawns are cooked through and the vegetables are tender.
Take the pan off the heat and add a splash of fish sauce and a squeeze of lime juice to taste.
Serve on a bed of rice (I like to garnish with sliced red chilli and coriander leaves) and lime wedges for squeezing over.