Chill the mixing bowl, whisk, and heavy cream in the refrigerator.
Soften the gelatin in cold water.
Juice the clementines to get 2/3 cup (about 150 ml) of juice.
Pour the juice into a saucepan with the sugar and heat. When it just starts to boil, add the drained and squeezed gelatin and stir until dissolved.
Let cool.

Whip the heavy cream with 1 tablespoon of sugar until stiff peaks form.
Gently fold in the cooled clementine syrup.
Transfer the mixture to a piping bag and refrigerate for at least 1 hour.

Fill the chocolate tartlet shells with the mousse once it starts to set.
Refrigerate for about 2 hours.
Before serving, sprinkle the mousse with cookie crumbs and decorate with a half slice of clementine.