Halfway fill a medium-sized pot with water. Place on its rim a metal bowl of the same size (bain marie process). Place the pot over medium heat and, in the bowl, melt the couverture pieces, add the margarine and the agave syrup and leave it until the chocolate melts completely. Turn off the heat, remove the bowl from the pot and mix with a silicone spatula until the mixture becomes homogeneous. Manually break the biscuits into very small pieces and add them to the melted chocolate mixture. Add the muesli to the mixture and stir until it becomes homogeneous. Empty the mixture into a long baking pan, spreading it uniformly with a spoon or spatula and put it in the refrigerator for 2 hours. Remove the dessert from the refrigerator and turn it over onto a serving platter. If you want to decorate the cake, you can grate a bit of white chocolate in a small bowl and sprinkle the flakes on top of the mosaic. Cut the dessert in 12 pieces using a sharp knife. If you want, cut each piece into small cubes.