Heat a splash of oil in a large saucepan and fry the onion until soft and translucent.
Add the garlic and ginger and cook for a couple of minutes, then stir in the red curry paste and tomato purée.
Pour in the stock, coconut milk, fish sauce, brown sugar and lime leaves, and bring the soup to the boil.
Reduce the heat to medium, add the salmon, pak choi, peppers and chilli, and let the soup simmer for about 5 minutes - or until the salmon is cooked through.Meanwhile, cook the noodles according to the packet instructions.
Season the soup to taste with salt, freshly ground pepper and lime juice, and add a pinch of chilli flakes if you would like it spicier.
Serve the soup over a bed of noodles, and top with sliced spring onions and chopped peanuts.