Peel 3 grapefruits so that the white skin is completely removed. Remove the fillets with a sharp knife between the parting skins. Squeeze the juice out of the parting skins and
Halve the remaining grapefruits and squeeze the juice, mix with agave syrup.
Spread the lady fingers side by side on the bottom of a casserole dish (approx. 17 x 27 cm). Sprinkle with half of the grapefruit-agaves mixture. Soak gelatine in cold water. Stir mascarpone, quark, vanilla sugar, 80 g sugar and the rest of the grapefruit and agave mixture until smooth.
Heat the cream in a small pot. Squeeze the gelatine and dissolve in the cream. Stir the cream into the mascarpone cream. Fill the cream onto the lady fingers and smooth it down. Chill for about 1 hour.
Cut the fillets in half. Heat juice and 30 g sugar in a small pot. Stir starch with 2 tbsp. water until smooth and bind the juice with it, bring to the boil. Add the fillets and spread on the tiramisu. Dust with cocoa.