the egg whites so they form stiff peaks. any less and the filling will collapse. if you need it, a pinch of cream of tartar might help. the cream should also be beaten so that it holds peaks. chilling overnight is highly recommended.
in a large bowl, beat the egg whites with an electric mixer until they form soft peaks. add the cream of tartar if you are using it. gradually add the sugar and continue to beat until they hold stiff peaks.
set aside ½ cup of juice from the frozen strawberries before beating the berries and remaining juice into the egg whites. continue beating until the mixture becomes fluffy and 