Wash the chard and separate the leaves from the stems and cut into fine strips, stems into small pieces. Cut onion into fine rings, garlic into thin slices. Remove the seeds from the chilli pepper and chop finely.
Melt butter in a saucepan, sauté onions, garlic and chard stems over low heat for 3 minutes. Add the chard and let it collapse. Stir in the cream and chilli and simmer for 5 minutes. Season to taste with salt.
Cook the spaghetti in salted water until al dente, drain and fold in the chard feta cream.