First, use a sharp knife to cut off the ends of the watermelon. You can then stand it up on either end for easier cutting. Cut the whole watermelon in half if you'd like.
Carefully cut the rind off of the watermelon and discard rind pieces.
Then, chop the watermelon into roughly 1-inch pieces. Be aware that the pieces may shrink while cooking.
Lay the watermelon pieces out on a large plate, making sure they aren't on top of one another.
Sprinkle 1 teaspoon of kosher salt over the pieces and allow them to sit for at least 5 minutes. This will help remove excess moisture from the watermelon.
While the watermelon is sitting, make the marinade.
To a bowl add the soy sauce, rice vinegar, toasted sesame oil, nori flakes, neutral oil, and lime juice and gently whisk together.
Once the watermelon cubes have sat for at least 5 minutes, pat them dry with a paper towel.
Add all the cubes to a sealable container or plastic sealable bag and cover with the marinade.
Seal the container and very gently shake to fully coat the cubes in the marinade.
Refrigerate for at least 2 hours, up to overnight.
Pre-heat your oven to 350º Fahrenheit.
After it has marinated, dump the entire container (including the excess marinade) onto a foil-lined baking sheet.
Spread the pieces out, making sure none are overlapping.
Roast in the oven for 50-60 minutes. Toss using a spatula halfway through roasting.
After the 50-60 minutes, remove from the oven.
Scoop the pieces onto a plate and place in the fridge to chill for about 20 minutes, or until cool.
Serve this vegan tuna on rice, in salads, in sushi, or more. Garnish with scallions, sesame seed, and/or spicy mayonnaise.