Preparation

• To cook pasta, bring a large pot of water to a rapid boil.
  Meanwhile, yank off any hairy beards protruding from mussel shells, then rinse shells under cold running water. Heat butter in a large wide saucepan set over medium heat. Add onion and garlic and sauté, stirring often, just until onion softens a bit, about 2 min. Then add tomatoes, wine, tarragon, chili flakes and salt. Increase heat to high and boil, uncovered and stirring occasionally, until wine is almost evaporated, about 8 min.
  
  Add pasta to boiling water and stir until all strands are separated. Then boil rapidly, uncovered and stirring occasionally, until al dente, from 8 to 10 min. Meanwhile, add all of mussels to onion mixture. Cover, reduce heat to medium and cook until most mussels have opened, about 6 min. Stir spinach into mussel mixture just until it starts to wilt, no more than 2 minutes. Discard any mussels that haven't opened. Add well-drained pasta to pan and toss until coated with sauce. Serve immediately.