Steam zucchini until very tender; drain in cheesecloth lined colander; squeeze out excess juice.
Mash and beat until smooth; drain again in colander.
Add butter, onion, salt, sugar, pepper, and egg yolks.
Mix well and beat.
Fold in 3 stiffly beaten egg whites.
Spoon carefully into individual oiled souffle molds.
Sprinkle with crumbs.
Bake at 375Â° for 25 minutes or until set and puffed.
Serve at once.