Melt 1 tablespoon of butter in a skillet over medium heat. Sauté the minced shallot for about 3 minutes, until it's translucent and fragrant.

Increase the heat to medium-high. Add garlic and fennel, seasoning with salt and pepper. Cook for 2-3 minutes, stirring, until everything is softened and aromatic.

Raise the heat to high and pour in the dry white wine and sherry vinegar (or Vermouth). Add fresh tarragon, bring to a boil, and allow it to reduce by half, approximately 3 minutes.

Carefully add the cleaned mussels to the pot. Stir well, cover, and steam for 5-7 minutes until the mussels open. Discard any mussels that remain closed after cooking.

Remove from heat and transfer the opened mussels to a bowl. Stir in the remaining cold, cubed butter and crème fraîche into the pan, swirling to emulsify. Return the mussels to the pot, garnish with fresh chives, and serve alongside the baguette.