How to Make It
Heat a heavy pot over medium-high. Add leek (or desired aromatics) and 1 tablespoon oil. Cook, uncovered and stirring often, until leek begins to soften, about 3 minutes.
Add mussels, stock (or other liquid), thyme, and bay leaf (or desired herbs). Cover and cook until mussels have opened completely, 3 to 5 minutes.
Discard any mussels that have not opened. Toss with tarragon (or desired garnish) and remaining 1 tablespoon oil. Serve with bread, if desired.