Whisk together 2 tablespoons chives, eggs, sour cream, and mustard in a large bowl.
Melt butter in a large oven-proof skillet over medium heat. Reduce heat to low; add egg mixture. As egg mixture starts to cook, gently lift edges with a spatula and tilt pan so uncooked portion flows underneath. Cook 6 to 8 minutes or until uncooked egg mixture no longer flows underneath cooked mixture. Remove from heat. Sprinkle with salmon, cream cheese, and onion.
Broil frittata, 3 inches from heat, 3 minutes or until set and cream cheese begins to melt. Remove from oven, and sprinkle with remaining chives. Cut into wedges, and serve immediately.