Mix the gelatine with half cup water in a small pan. Cook over low heat, stirring continuously, till it dissolves.
In a bowl, beat the condensed milk till light and creamy. In another bowl, mix the lemon juice with half a cup of pineapple syrup from the tin.
Chop five slices of pineapple. Add the pineapple syrup and pineapple to the condensed milk. Pour the gelatine solution into the condensed milk mixture stirring continuously.
Freeze the mixture for half an hour till it is thick. Remove from the freezer and beat till smooth. Fold in two hundred grams of cream and reserve the remaining cream for decoration.
Add the pineapple essence and yellow colour and beat well and put it back in the freezer for fifteen to twenty minutes.
Remove from the freezer and beat again till smooth. Pour into a serving dish and freeze for one hour or till set.
Whip the reserved cream with powdered sugar. Remove the mousse from the freezer. Put the whipped cream in a piping bag fitted with a star nozzle and pipe rosettes on the surface of the mousse.
Place a glace cherry on each rosette and serve immediately.