Roasted Black Bass with Mussels Panzanella

Name: Roasted Black Bass with Mussels Panzanella
Ingredients: 2 pounds small mussels, preferably Prince Edward Island, scrubbed well and debearded; 1 cup dry white wine; 1/2 teaspoon black peppercorns; 1 1/2 cups fresh basil, any combination of regular, purple, or Thai, stems reserved, plus more for serving; 6 slices rustic Italian bread (1/2 inch thick); 1/4 cup extra-virgin olive oil, plus more for brushing and drizzling; 2 pounds mixed tomatoes, cut into 1/2-inch-thick rounds; 2 tablespoons balsamic vinegar; 1 tablespoon plus 1 teaspoon finely grated lemon zest; 1/2 teaspoon red-pepper flakes; Coarse salt and freshly ground pepper