Step 1

Heat oil in a medium pot over medium-high. Add fennel and shallots and cook, stirring often, until fragrant and beginning to soften, about 2 minutes. Add garlic, season with salt, and cook, stirring constantly, until fragrant, about 1 minute.

Step 2

Add mussels and stir gently once or twice to coat with oil. Pour in beer and stir once more to coat. Cover pot and steam mussels, stirring halfway through, until they open, about 3 minutes. (Discard any mussels that do not open.) Add butter to pot and mix until butter is melted into pan sauce and mussels are coated.

Step 3

Transfer mussels and sauce to a serving bowl and top with tarragon. Serve with bread alongside for dipping.