Bring water to a boil in a large saucepan and add salt. Add salmon fillet, reduce to a simmer, and cook until just cooked through, 6 to 8 minutes. Transfer salmon to a plate using a slotted spoon and let cool for 10 minutes. Flake salmon into a large bowl, discarding skin, and toss with smoked salmon, green onion, parsley, and tarragon.

Whisk together butter, mayonnaise, mustard, lemon juice, and pepper in a bowl until smooth. Add mayonnaise mixture to salmon mixture and gently stir until well combined.