Perheat the oven to 190 C / Gas 5. Grease a 20cm (8 in) springform cake tin, then line with baking parchment.
Place the crushed biscuits in a bowl. Lightly beat 1 of the egg whites until frothy and add to the biscuits. Stir well, then spoon the mixture into the prepared tin and press evenly over the bottom.
Bake in the oven for 15 minutes, then set aside to cool.
Dissolve the gelatine in the boiling water. In a separate bowl, combine the quark, yoghurt, sweetener and vanilla and whisk till smooth.
Whisk the 2 remaining egg whites until stiff, then fold into the quark mixture along with the gelatine liquid. Spoon over the prepared biscuit base, smooth the top and chill for 4 to 5 hours, or until set.
Remove the pie from the tin and place on a serving plate. Slice the bananas and arrange neatly on top of the pie.