Vegan watermelon "tuna" sushi is made by roasting watermelon in the oven with sake and nori and letting it cool. Once cool, the watermelon slices just like tuna.
Cut the rind off the watermelon and then cut it into 1.5-2” thick slices.
On a parchment lined sheet pan, place the watermelon slices on top of the nori sheet. Pour the sake over the watermelon and then bake in the oven for 3 hours.
Let the watermelon cool and discard the nori sheet. Pour off any excess water. Slice into your desired “tuna” slices or dice for tartare.