Preheat the oven to 400 degrees.
Butter the insides of 4 individual souffle molds.
Place buttered molds in the refrigerator.
Saute the onion in the butter in a skillet until tender.
Beat the egg yolks in a medium mixing bowl until very thick and light in color.
Beat in the cream cheese, sour cream, mustard, smoked salmon and sauteed onion.
Beat the egg whites with the lemon juice in another medium bowl until firm but still glossy.
Fold the egg white mixture into the egg yolk mixture.
Spoon into the prepared souffle molds.
Bake for 8 to 10 minutes or until puffy and browned.