Combine salmon, onion, if desired, dill and lemon juice in medium bowl. Mix until well combined. Add cream cheese and mash with fork until mixture is smooth. Season to taste with salt and pepper.

Stack celery stalks in pairs. Cut each pair into 3-inch pieces.

Spread 2 tablespoons salmon mixture into hollowed section of each celery piece with small spoon or knife. Press dill springs into one half of each celery pair before pressing filled sides together. Stand upright on serving platter with dill sprigs on top to resemble trees with branches.