Heat chorizo and 2 tablespoons oil in a large heavy pot over medium, stirring occasionally, until chorizo begins to brown and crisp, about 4 minutes. Add garlic and fennel seeds and cook, stirring, until fragrant, about 1 minute. Add tomatoes and wine and bring to a simmer; season with pepper. Cook until reduced by three-quarters, 6–8 minutes.
Add mussels. Cover and cook, stirring occasionally, until mussels open, 6–8 minutes. Discard any mussels that don't open. Drizzle toast with oil. Serve mussels topped with tarragon with toast alongside.
