Step 1

Trim dark green tops and root ends from leeks, leaving ends intact. Cut leeks in half lengthwise; peel off tough outer leaves. Wash leeks; pat dry. (Keep some water on leeks to prevent burning on the grill.) Brush with 1 Tbsp. oil; season with 1/4 tsp. salt and 1/4 tsp. black pepper.

Step 2

On a grill or grill pan over medium-high heat, grill rosemary sprig 1 to 2 minutes or until lightly charred; remove. Grill the leeks 5 to 7 minutes or until tender, turning occasionally. Remove; cover to keep warm.

Step 3

Brush salmon with remaining olive oil; season with 1/4 tsp. each salt and black pepper. Grill fish, skin-side up, 4 minutes; turn. Grill 2 minutes or until fish flakes easily with a fork. Remove from heat.

Step 4

Strip rosemary leaves from stem; chop leaves. In a small bowl stir together butter, mustard, and 1 tsp. chopped leaves. Spread butter on top of fish and leeks; sprinkle with remaining chopped rosemary.