Heat the olive oil in a large saucepan or Dutch oven over medium-high heat.

Add the fennel, celery, and garlic.

Cook, stirring occasionally, until slightly tender, about 3 to 4 minutes.

Add the white wine and bring to a boil.

Add the cod, shrimp, and tomatoes.

Cook, covered, for 3 minutes.

Add the mussels.

Cook, covered, until the cod and shrimp are cooked through and the mussels have opened, about 3 to 5 minutes.

Stir in the red wine vinegar, salt, and pepper.

Sprinkle with the fresh tarragon before serving.