Roast Cod with Potatoes, Onions, and Olives

INGREDIENTS

• 1/2 teaspoon dried rosemary, crumbled
• 1/3 cup vegetable oil
• a 2 1/2-pound piece of center-cut cod fillet
• 1/4 cup minced fresh parsley leaves
• 3 large garlic cloves
• 1 pound (about 2 large) boiling potatoes
• 3/4 cup Kalamata or other brine-cured black olives
• 1 pound (about 3) onions, cut into 1/2-inch-thick slices
• 1/4 cup olive oil