Cut the bananas into 3 lengthways slices and place them side by side in a large, flat baking dish. The bottom of the form must be completely filled. Spread the lemon zest and brown sugar on the bananas. Add rum to the cream and pour over the bananas. Scatter the coconut flakes over the gratin. Bake in a preheated oven at 180 degrees for 15-20 minutes.