Bring a medium saucepan of water to the boil, and add the pasta. Reduce to a simmer and allow to cook for 8 to 10 minutes (or according to the packet instructions).
Meanwhile, peel and finely slice the onion, and very finely chop the chilli and parsley. Put into a frying pan with the oil, lemon zest and juice and cook over a medium heat to soften the onion.
When the pasta is cooked, remove from the heat and drain.
Quickly stir the yoghurt and fish paste into the onions to warm through. Tip the pasta into the pan and stir to coat with the sauce. Serve, garnishing with extra chopped parsley and lemon juice if desired.