Mix 250 ml whipping cream (25% fat) with half a lemon using a whisk; let stand 30 minutes and add salt; refrigerate.
Add grated cucumber and finely chopped mint to the sour cream; mix and set aside.
Combine balsamic vinegar, soy, and sesame oil in a bowl; place watermelon pieces and macerate for at least 1 hour.
Brown the pieces on all sides in a greased pan.
Coat with a sesame seed and crushed seaweed mixture.
Cut into 1 cm slices and serve with tzatziki sauce.
Sprinkle mint over the sauce.