Baked Cod with Olives

Name: Baked Cod with Olives
Ingredients: Unsalted butter, for baking dish; ½ cup pitted brine-cured Kalamata olives, drained and chopped; 2 teaspoons drained capers, rinsed and coarsely chopped; 2 anchovy fillets, rinsed, drained, and coarsely chopped; 1 garlic clove, minced; 1 teaspoon fresh lemon juice, plus several thinly sliced lemon rounds, halved if large; 3 tablespoons extra-virgin olive oil; 1 tablespoon chopped fresh oregano, plus small leaves for garnish; 4 center-cut cod fillets (6 to 8 ounces and 1 inch thick each), skinned; Coarse salt and freshly ground pepper