Toast sesame seeds in a small frying pan over medium heat until golden 3 to 5 minutes, shaking pan often.
Add butter, onion, sherry, and soy sauce; cook, stirring, until butter is melted.
Remove from heat.
Rinse fish and pat dry.
Place on greased rack of a large broiler pan spread baby salmon open and place skin sides down.
Brush with butter mixture.
Broil about 4 inches below heat until just opaque but still moist in thickest part; cut to test 6 to 8 minutes.