Soak gelatine in cold water. Rinse the box form (25 cm; approx. 1.5 litres capacity) with water and line it with cling film. Peel orange and grapefruit so that the white skin is completely removed.
Remove the fillets with a sharp knife between the parting skins, collecting the juice. Press the juice out of the parting skins and fill up with orange juice at 1⁄4 l. Cut the fillets in half. Wash the mint, shake dry, pluck off the leaves and chop finely.
Separate eggs. Whisk egg yolk and sugar until white-creamy. Squeeze the gelatine and dissolve in a saucepan over low heat. First stir in 2 tbsp. mascarpone, then stir into the remaining mascarpone. Fold the mascarpone, mint, orange and grapefruit pieces into the egg-sugar mixture.
Beat the egg white and 1 pinch of salt until stiff and fold in.
Soak each biscuit briefly in the juice. 1⁄3 of the biscuits into the mould. Spread half of the cream on top. Add another 1⁄3 of the biscuits. Spread the rest of the cream on top and cover with the remaining biscuits.
Cover and leave to cool for approx. 4 hours. Turn out of the mould and remove the foil. Cut the tiramisu into slices and serve.