Heat oil in a pot. Add garlic, fennel and saffron. Cook until fragrant, about 1 minute.
Add tomatoes, wine, and red pepper flakes. Bring up to a simmer and reduce by 75%, about 6-8 minutes.
Clean the mussels and remove beards. Add mussels and tarragon to simmering broth. Cover and cook, stirring occasionally until mussels open, 6-8 minutes. Discard any mussels that don't open and serve.