Sprinkle halibut fillets with salt and pepper. Heat 2 tablespoons olive oil in heavy large skillet over medium-high heat. Add halibut fillets and saute until just opaque in center, about 4 minutes per side. Transfer to plate; cool. Wipe skillet with paper towels. Flake halibut fillets into large bowl. Mix in green onions, sardines, chopped fresh parsley, flour, garlic, and lemon peel. Mix in 1 cup breadcrumbs. Season with salt and pepper; mix in egg. Shape into six 3-inch-diameter cakes. Transfer remaining 1 1/2 cups breadcrumbs to bowl. Coat fish cakes with breadcrumbs, pressing to adhere. Heat remaining 4 tablespoons oil in reserved skillet over medium-high heat. Add fish cakes and cook until brown and crisp, about 6 minutes per side. Transfer to plates; serve fish cakes with Caper-Parsley Sauce.