Bass in Artichoke and Tomato Broth

Name: Bass in Artichoke and Tomato Broth
Ingredients: 1 small onion, quartered and thinly sliced; 4 tablespoons extra-virgin olive oil; 2 garlic cloves, chopped; 1/3 cup dry white wine; 1 (9-ounce) package frozen artichoke hearts, thawed and drained; 1 (15-ounce) can stewed tomatoes; 1/3 cup pitted brine-cured black olives such as Kalamata, chopped; 2/3 cup water; 1 1/4 teaspoons salt; 1/2 teaspoon black pepper; 4 (6- to 7-ounces) black sea bass fillets (1/2 inch thick) with skin; 1 tablespoon unsalted butter, cut into bits; 1 teaspoon finely grated fresh orange zest