Heat a large frying pan over medium heat.
Add the olive oil and 10 g butter.
Once the butter begins foaming, add the finely diced shallot.
Cook for approximately 2–3 minutes, until softened but not heavily browned.
Add garlic
Add the garlic and optional chilli flakes.
Cook for about 30 seconds, stirring continuously.
You want the garlic fragrant but not browned.
Add the white wine
Pour in the 150 ml white wine.
Increase the heat to medium-high and bring to a strong simmer.
Scrape the bottom of the pan to release anything that has started sticking.
Let the wine reduce by approximately half, which should take around 3–5 minutes.
This step is important.
If you add the mussels immediately after adding the wine, they'll be sitting in the pan while you're waiting for 150 ml of alcohol and water to reduce. That's exactly how already-cooked mussels become rubbery.
Add the mussels
Once the wine has reduced, add the drained mussel meat.
Add the black pepper.
Toss gently through the sauce.
For pre-cooked mussels, cook for approximately 2–3 minutes, just until they're thoroughly heated.
Don't keep simmering them once they're hot.
Finish with cold butter
Turn off the heat.
Add the remaining 15 g cold butter.
Gently toss or swirl the pan as the butter melts into the reduced wine and mussel juices.
The sauce should become slightly glossy while remaining quite light.
This isn't supposed to become a thick butter sauce.
Add lemon
Start with 1 teaspoon lemon juice.
Taste.
Add the second teaspoon if you think it needs more acidity.
Don't automatically add salt. Mussels can contribute considerable salinity themselves, so taste the finished sauce first.
Add spring onion
Add most of the finely sliced green spring onion and toss briefly.
Transfer everything to a shallow serving bowl.
Finish with the remaining spring onion and another crack of black pepper.
Serve immediately with lemon wedges and plenty of crusty bread.