Grease the inside of a 3- to 4-quart
slow cooker with butter. In a large
bowl, using a handheld mixer, beat
the egg whites on high speed until
soft peaks form. In another large
bowl, beat together the egg yolks,
yogurt, lemon peel and lemon juice
on medium speed for 1 minute. Add
the sugar, flour and salt and beat on
medium speed until smooth, about
2 minutes. Working in 3 or 4 batches,
gently fold in the beaten egg whites.
Scatter three-quarters of the
banana slices on the bottom of the
slow cooker, then pour in the batter.
Cover and cook on high until the
cake is puffed, the top is set and the
edges are browned, about 1 hour
20 minutes. Remove the slow cooker
insert, still covered, and cool for
about 20 minutes.
Spoon the pudding into bowls and
top with the whipped cream, if using,
and the remaining banana slices.