For a firmer set, dissolve 1 tsp unflavored gelatin in 1 tbsp cold water, let bloom 5 minutes, then warm gently until melted and whisk into the filling before folding in the whipped cream. To prevent banana browning on top, lightly brush slices with lemon juice. Store covered in the refrigerator for up to 3 days or freeze (without fresh banana topping) for up to 1 month.