Cut a piece of aluminum foil long enough to fit around a 1 1/2 quart souffle dish, allowing a 1 inch overlap; fold foil lengthwise into thirds.
Lightly butter one side of foil and bottom of dish.
Wrap foil around dish, buttered side against dish, allowing it to extend 3 inches above rim to form a collar.
Secure foil with string.
Set aside.
Melt butter in a heavy saucepan over low heat; add flour, stirring until smooth.
Cook 1 minute, stirring constantly.
Gradually add milk; cook over medium heat, stirring constantly, until mixture is thickened and bubbly.
Stir in salt, pepper, and onion; remove from heat, and let cool.
Squeeze grated zucchini to remove as much liquid as possible; stir into sauce.Beat egg yolks until thick and lemon colored; add to squash mixture, and stir well.
Beat egg whites and cream of tartar until stiff, but not dry; fold into squash mixture.
Pour into prepared dish.
Bake at 350° for 1 hour and 10 minutes; sprinkle with cheese, and bake an additional 5 minutes or until a knife inserted in center comes out clean.