Heat up a large sauté pan or wide pot over medium high heat.
Add butter and oil and allow butter to melt.
Add leeks and cook for about three minutes.
Add garlic and chili flakes and season with salt and pepper.
Allow garlic to cook for about a minute and add the mussels.
Toss them to coat with leek and butter and then add the white wine.
Cover with a tight-fitting lid and cook for three minutes. Remove lid, you should notice a few of the mussels are open — this is a good sign.
Turn up the heat and add the cream.
Reduce the cream a touch until a nice sauce consistency is achieved.
Taste for seasoning and adjust accordingly.
Once all the mussels have opened you are ready to eat. Discard any mussels that didn't open. Serve with bread.