• 2 1/2 pounds red and yellow bell peppers (about 5 total)
• 3 tablespoons extra-virgin olive oil plus more for grilling
• 1 leafy sprig marjoram plus 1 tablespoon whole fresh leaves
• Coarse sea salt
• Freshly ground black pepper
• 2 medium tomatoes
• 1/2 cup finely chopped onion
• 12 fresh sardines (about 2 to 3 ounces each), cleaned, leaving head and tail intact
   

Char peppers over a gas burner set on high, turning frequently, until skin is blackened and blistered on all sides. Transfer to a bowl and cover tightly with plastic wrap; let sit for 15 minutes.

Rub skin off peppers (do not rinse under water); discard core and seeds. Cut peppers into 1 ⁄ 3-inch square pieces. Combine in a bowl with 2 tablespoons oil, marjoram sprig, and generous pinch salt and pepper. Let sit for 15 minutes at room temperature.
Meanwhile, bring a large saucepan of salted water to boil. Drop tomatoes into water and boil 30 seconds. Using a slotted spoon, remove tomatoes. Peel, seed, and cut into 1-inch pieces.

In a large nonstick skillet, heat remaining tablespoon oil over medium-high heat. Add onion, reduce heat to medium and cook, stirring occasionally, until softened, about 5 minutes. Remove marjoram sprig from pepper mixture. Add peppers and tomatoes to pan. Bring peperonata to a simmer and cook, stirring occasionally, until peppers are tender and flavors have blended, about 15 minutes. Remove from heat. Adjust seasoning.

Heat grill pan over moderately high heat until hot. Drizzle fish with oil; grill, turning once, until just cooked through, 4 to 5 minutes. Sprinkle with salt. Spoon peperonata into a large serving dish; top with fish and marjoram leaves. Serve warm.