Heat butter in a medium nonstick skillet over medium heat. Add leek; cook 5 minutes or until tender, stirring occasionally. Add water and peas; cook 5 minutes or until peas are tender.
Place pea mixture in a food processor. Add basil, juice, and tarragon; process until smooth, adding more water if necessary. Stir in 1/4 teaspoon salt and 1/4 teaspoon pepper; keep warm.
Sprinkle salmon with remaining 1/4 teaspoon salt and remaining 1/4 teaspoon pepper. Heat a nonstick skillet over medium-high heat. Coat pan with cooking spray. Add salmon, skin side down, and cook for 6 minutes or until golden. Turn, and cook for 8 minutes or until fish flakes easily with a fork or until desired degree of doneness. Serve salmon over warm mashed peas. Garnish with lemon wedges, if desired.