Using a sharp knife, slice off the tops and bottoms of the tomato. Locate the fleshy dividing wall of one segment inside the tomato. Slice into the wall and open up the flesh to expose the seeds. Remove the seeds and their pulp by slicing through the tomato the core. Set aside the seeds. Your aim is to keep the pulp of the seeds together to create tomato seed "fillets" that are separate from the firmer tomato flesh. Repeat with each segment of the tomato.
Using a small melon baller, hollow out the watermelon cubes to create little watermelon cups.
Place 3 watermelon cubes on each plate. In cube #1, spoon 1/2 teaspoon of double cream and top with 1/2 teaspoon of the roe. In cube #2, add a few drops of balsamic vinegar. Using a microplane grater, shave a generous amount of Parmesan over top. In cube #3, place a tomato seed fillet.