Cioppino with Parsley and Olive Gremolata

For the Parsley-Olive Gremolata (Optional)

• 1/2 bunch parsley, about 2 ounces, roughly chopped
• The zest from one orange
• 1/2 cup mixed olives, pitted
• 1 clove garlic, peeled and roughly chopped
• 1/2 teaspoon kosher salt
• 1/2 teaspoon red pepper flakes, optional
• 2 tablespoons extra-virgin olive oil

For the Cioppino

• 1 pound clams, scrubbed well
• 1 pound mussels, debearded and scrubbed well
• 1/4 cup extra-virgin olive oil
• 1 large onion, cut into ½-inch dice (about 2 cups)
• 1 large fennel bulb, cut into ½-inch dice (about 2 cups)
• kosher salt, to taste
• 2 cloves garlic, finely minced
• 1 12 ounce jar roasted red peppers, drained and roughly chopped
• 2 teaspoons dried oregano
• 2 teaspoons dried thyme
• 1 ¼ cup dry white wine
• 1 28 ounce can whole tomatoes
• 1 ¼ cup seafood stock
• 1 pound skinless firm white fish, such as halibut or cod, cut into 1-inch pieces
• 1 pound large shrimp, peeled and deveined
• Grilled sourdough bread for serving, optional