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.
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.
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.

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Nutrition Facts

Per Serving Per 100 g

Amount Per 1 Serving
Calories	1266.86 Kcal (5304 kJ)
Calories from fat	218.92 Kcal
		% Daily Value*
Total Fat	24.32g	37%
Cholesterol	15.27mg	5%
Sodium	2336.15mg	97%
Potassium	883.67mg	19%
Total Carbs	225.74g	75%
Sugars	17.84g	71%
Dietary Fiber	15.32g	61%
Protein	37.94g	76%
Vitamin C	32.1mg	54%
Vitamin A	0.1mg	2%
Iron	0.2mg	1%
Calcium	86.4mg	9%

Amount Per 100 g
Calories	167.27 Kcal (700 kJ)
Calories from fat	28.9 Kcal
		% Daily Value*
Total Fat	3.21g	37%
Cholesterol	2.02mg	5%
Sodium	308.45mg	97%
Potassium	116.67mg	19%
Total Carbs	29.81g	75%
Sugars	2.36g	71%
Dietary Fiber	2.02g	61%
Protein	5.01g	76%
Vitamin C	4.2mg	54%
Calcium	11.4mg	9%

* Percent Daily Values are based on a 2000 calorie diet. Your daily values may be higher or lower depending on your calorie needs.
Find out how many calories should you eat.