Step 1

Heat broiler. Cut pointed ends off lemon, halve crosswise, and place on a rimmed baking sheet, center cut sides up. Broil on top rack until charred, 5 minutes; transfer to a plate and set aside.

Step 2

Reduce oven temperature to 400°F. On rimmed baking sheet, toss fennel and onions with 1 1/2 Tbsp oil and 1/4 tsp each salt and pepper; arrange around edges of sheet. Place salmon in center of sheet and season with 1/4 tsp each salt and pepper. Roast until vegetables are tender and salmon is opaque throughout, 17 to 20 minutes.

Step 3

Juice charred lemon halves into a small bowl and whisk in mustard and remaining Tbsp oil. Remove baking sheet from oven and fold arugula into vegetables. Drizzle charred lemon vinaigrette over fish and vegetables and gently toss vegetables.