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.