Rinse salmon; pat dry with paper towels. Place salmon in a large resealable plastic bag set in a shallow dish. For marinade: In a medium bowl, stir together the water, cider, and kosher salt until salt dissolves. Stir in snipped tarragon and pepper. Pour over salmon; seal bag. Marinate in the refrigerator for 4 hours, turning bag occasionally.
Preheat broiler. Line a baking sheet with foil; lightly coat foil with nonstick cooking spray. Drain salmon, discarding marinade. Pat salmon dry with paper towels. Discard brine. Place salmon, skin side down, on prepared baking sheet. Brush salmon with olive oil. Broil 5 to 6 inches from the heat for 5 to 7 minutes or until fish flakes when tested with a fork. Remove from broiler; and cover with foil to keep warm.
In a small saucepan melt butter over medium heat. Add shallot; cook and stir for 4 to 5 minutes or until tender. Stir in flour. Cook and stir for 1 minute. Add wine, cream, mustard and 1/2 teaspoon salt, whisking until smooth. Cook and stir until thickened and bubbly; cook and stir 1 minute more. Remove from heat. Place salmon on a serving platter. Drizzle with sauce. If you like, garnish with tarragon springs. Makes 4 servings.