Unfortunately, you can’t fix an assembled soupy tiramisu because of all of the different ingredients that are in it. To prevent it: Make sure that the egg yolks and sugar is cooked until it becomes thick and feels very hot to the touch (careful!). Let it cool down before adding the mascarpone cheese. A good quality, full fat mascarpone cheese will keep the filling thick. Make sure the cream is whipped until medium-stiff peaks. Keep the cream, bowl, and whisk cold to help it whip well. As long as you follow all of the steps you should get a thick, mousse-like filling! The tiramisu needs to set for at least 8 hours or overnight for it to thicken even more.