In a medium saucepan, melt butter, and stir in flour. Cook for 1 minute, over low heat, stirring. Slowly pour in milk, stirring for a few minutes. Remove from heat. Separate the eggs, placing all egg whites in a mixing bowl. Reserve the yolks. Beat the whites until stiff but not dry. Beat the yolks, sugar and lemon rind together until creamy. Combine egg yolks with the saucepan mixture. Add diced banana. Fold in stiff egg whites, using a large bowl if saucepan is too small. Spoon into lightly greased souffle dishes. Bake at 350 degrees for 30 to 35 minutes until firm. Serve with strawberries.