- Preheat the oven to 350 degrees F.
- Remove the peel from the watermelon with a peeler. Reserve 6 long pieces for garnish if desired.
- Cut the rind off and very finely julienne 2 cups with a mandoline. Place in ice water to crisp. Set aside.
- Cut the flesh into 1” pieces.
- Coat the bottom of a baking dish with rice oil and drizzle the watermelon pieces with the rest. Bake "roast" for 20-25 minutes.
- After 20 minutes, turn the watermelon over and bake for another 10 minutes, watching to make sure it doesn’t brown. Remove from the oven to cool slightly.
- Meanwhile, combine the tamari, mirin, and rice vinegar in a shallow dish.
- When the watermelon has cooled slightly, add the watermelon, cover, and let it rest for 20 minutes Chill refrigerated for 1 hour.
- After 1 hour, remove watermelon from the marinade, dry well and slice into ½ inch slices. Strain and reserve the marinade.
- Drain the watermelon rind.
- To assemble, place 1½ cups of sushi rice at the bottom of each bowl.
- In a decorative pattern, place watermelon rind, 1 shiso leaf, 3 prawns, ½ of a sliced cucumber, some seaweed, 2 slices of tamago, some pickled ginger and wasabi paste.
- Shingle 1 roasted sliced watermelon sashimi in the center.
- Sprinkle with furikake, decorate with the reserved watermelon peel, if desired, and serve with reserved marinade on the side.