Peel and core the apple and cut them into quarters.
Grate the apples with a cheese grater. If your grater is not able to grate the fruit into strips, cut the apple by hand instead. You can choose to cut them into strips or even cubes, but if cutting into cubes, it is best to cut them into small cubes to not overpower the mousse.
Place the grated apple into a small saucepan. Add lemon juice and mix well.
Add the brown sugar and cinnamon powder to the apple and lemon juice mixture.
Turn on the heat to a medium low flame and cook the apples until they turn soft and the mixture thickens. As you cook the apples and the sugar, there will be liquid released in the pot. Continue cooking until the apples turn soft and the liquid dries up (see Note 1).
Once ready, turn off the heat and let the compote cool down to room temperature. Place it in the refrigerator to chill completely before assembling the apple mousse.
In a separate heat proof bowl, break the white chocolate into small pieces (see Note 2). Place them in a heatproof bowl and add 100ml of the cream.
Double boil the white chocolate and the cream until the chocolate is all melted. Stir to expedite the melting and to ensure the cream and the chocolate combine well (see Note 3).
Set aside to cool completely to room temperature. This chocolate mixture need not be refrigerated to avoid it from hardening.
In a separate bowl, add the remaining cream and whip it on high speed until soft peaks form. Do not overwhip (see Notes 4 & 5).
Transfer about 2 spoonful of the whipped cream into the completely cooled white chocolate. Fold in the cream until well combined.
Repeat the step with a few more spoons of the whipped cream. Fold well, but do not over mix.
Finally, transfer all the white chocolate mixture into the remaining whipped cream and fold until the chocolate is well combined.
Transfer about a quarter of the white chocolate mouse into a piping bag fitted with a large star tip.
To assemble the mousse, add the chilled apple compote into the remaining mousse (see Note 6). Fold it in gently to combine well. Do not over mix.
Transfer the apple mousse into a large piping bag.
Snip the tip of the piping bag (make sure the hole is large enough to let the apple pieces come through) and pipe the apple mousse into prepared mousse cups.
Tap the cups on your kitchen counter or table to level the tops. Tap the cups on a cloth if necessary to avoid them from breaking.
Pipe the reserved white chocolate mousse (in the piping bag fitted with a star tip) into swirls on top of each apple mousse cup and decorate with a small apple slice each.
Chill completely for at least one or two hours before serving to allow the mousse to set.