I think I found this in Food & Wine Magazine about 3 years ago. Quite tasty. Make sure you have a big loaf of crusty bread to go with this to sop up all the great broth.
In a large pot, heat the olive oil. Add the garlic, onion, bell pepper, tomatoes, oregano and cayenne and cook over moderately high heat, stirring occasionally, until softened, about 5 minutes.
Add the Sambuca and boil for 1 minute.
Add the mussels, cover and cook until the mussels open, about 5 minutes.
Transfer the mussels to 6 bowls. Return the pot to high heat and stir in the feta and butter. Pour the broth over the mussels and serve.
NOTE: You may also put the mussels into a large bowl and set in the middle of the table and let everyone serve themselves. Don't forget the crusty bread!