Scrub and debeard the mussels. Discard broken shells and any open mussels that do not close when tapped. Keep chilled until cooking.
Heat the 50 g butter and olive oil in a large lidded pot over medium heat. Soften the onion for 4–5 minutes without browning. Add garlic and chilli flakes and stir for 30 seconds.
Turn the heat high, pour in the wine and boil for about 1 minute.
Add the mussels, cover and steam over high heat for 3–4 minutes, shaking the pot once or twice, until the mussels open and are cooked. Discard any that remain closed.
Take the pot off the heat. Add the cold butter, lemon juice, parsley and black pepper. Shake or stir gently for 10–15 seconds until the broth is glossy.
Taste before adding salt, as the mussels season the broth. Serve immediately with crusty bread.