Ingredients

• 12 butterflied fresh sardines or six 6-inch-long trout or branzino fillets with skin
• 2 tablespoons olive oil plus additional for brushing
• 1 tablespoon chopped fresh thyme
• 1 tablespoon chopped fresh Italian parsley
• 2 teaspoons grated lemon peel
• 1 teaspoon piment d'Espelette or 1/2 teaspoon dried crushed red pepper
• Nonstick vegetable oil spray
• 6 1/3-inch-thick slices sweet onion (such as Maui or Vidalia)
• 6 6-inch-long baguette slices, cut 1/2-inch thick on sharp diagonal
• 3 tablespoons butter, room temperature
• Fleur de sel*
• 18 arugula leaves
• 1 tablespoon extra-virgin olive oil
• 1/2 teaspoon fresh lemon juice
• 6 lemon wedges
• *A delicately flavored sea salt; available at some supermarkets and at specialty foods stores.

Instructions

1. Place fish, skin side down, in 13x9x2-inch glass baking dish. Whisk 2 tablespoons olive oil and next 4 ingredients in small bowl. Spread herb mixture over fish. Cover and chill at least 2 hours and up to 8 hours.
2. Spray grill with nonstick spray. Preheat barbecue (medium heat). Brush onion slices with olive oil; sprinkle with salt and pepper. Grill onions until just tender and slightly charred, about 2 minutes per side. Grill bread slices until slightly charred, about 1 minute per side. Spread 1 side of each bread slice with butter. Sprinkle fish with fleur de sel and pepper. Grill fish until just opaque in center, about 3 minutes per side. Remove bones from fish.
3. Place arugula in medium bowl. Drizzle with 1 tablespoon extra-virgin olive oil and lemon juice and toss; season to taste with fleur de sel and pepper. Arrange 3 arugula leaves atop each bread slice; top with grilled onions. Top onions on each bread slice with 2 sardines or 1 trout or branzino fillet, trimming fish to fit. Garnish with lemon wedges and serve.