“Tuna” Gunkan Maki Sushi (Vegan)

I’ve made the Watermelon Tuna multiple times in the past few weeks, but every time I do, it still blows my mind. These oven roasted, soy-marinated watermelons achieve a total change in texture and taste, making them a good substitute for tuna. Get seated, It’s time for some plant-based omakase!

Let’s get straight into what kind of magic I’ve done to the watermelon to transform them into tuna. The process involves three main steps: 1) Marinate. 2) Roast. 3) Marinate.

Okay, that’s only three words. But if you really took your time, you can do this for a few days, especially the marinating part (granted, the more you marinate, the more tender & flavorful your watermelon will become!). The initial marinade consists of soy sauce, sesame oil, mirin, and seaweed flakes — it’s meant to help you infuse all those familiar umami and briny flavors into the watermelon.

Roasting Watermelon

Yep, you read that right. We’re roasting the watermelon to give it that slightly squishy yet firm tuna texture. The roast is a very slow one — the low temperature will slowly tenderize the watermelon without charring or caramelizing it, so as to retain that raw look.

Before Roasting
After Roasting

The final marinade is a brief but important one. After a couple of trials, I realized that when you place the hot watermelon (after roasting) back into the cold marinade, you not only allow the watermelon to cool down quicker, but allow the watermelon to re-absorb all that umami flavor from the marinade.

Tuna Gunkan Maki Sushi

5 from 12 votes
Recipe by George L. Course: LunchCuisine: JapaneseDifficulty: Medium


Prep time


Cooking time



Oven-roasted, soy-marinated watermelon that achieves texture just like tuna.


  • Watermelon “Tuna”
  • 200 grams watermelon, cubed

  • 4 tablespoons soy sauce

  • 3 tablespoons sesame oil

  • 1 tablespoon mirin (or rice wine)

  • 1/2 teaspoon dried seaweed flakes

  • 1/2 teaspoon chili powder

  • Gunkan Maki Sushi
  • 1 cup rice

  • 1 tablespoon rice vinegar

  • 1 teaspoon sugar

  • Watermelon “Tuna,” roughly chopped

  • 1 piece nori, sliced into strips


  • Combine all the marinade ingredients and marinate the watermelons in the refrigerator for at least 6 hours (preferably overnight).
  • I actually forgot about them and left them in the refrigerator for 2 days… The color is still vibrantly red but I could smell that they’ve taken up all that umami flavor! Preheat the oven to 275 F (135 C) and place them on a parchment paper lined baking tray. Save most of the marinade.
  • Bake in the preheated oven for 1.5 hours to 2 hours. Make sure to use a spatula to turn them or move them around once every 30 minutes or so. In the end, the watermelon should become squishy and bouncy. At this point, place them back into the marinade while you prepare your other ingredients.
  • Now onto the sushi rice. Take some freshly cooked rice and mix with rice vinegar and sugar with a rice scooper. You should be doing this while the rice is still hot*
  • Make oval rice balls in your palm (dampened with water!) and set them aside to let dry.
  • Take your “tuna” out of the marinade and roughly chop them. At this point you can remove all the seeds that were buried within the watermelon cubes.
  • When everything is prepared, take out your nori and slice length-wise into strips. You want to do this rather fast*. Using dry hands, roll the nori strip around the oval rice balls, and close up the ends.
  • Spoon a healthy portion of the minced tuna — enjoy immediately!


  • Details about Sushi Prep. 1) The hot / warm rice will allow the sugar to melt and be incorporated nicely. 2) The reason we’re preparing the nori last is because it absorbs water from the air and becomes soggy very fast — keep the nori in its container as soon as possible!
  • Best Enjoyed Immediately. Since watermelon “tuna” has more moisture than real tuna, the nori will become soggy very fast once you finish assembling the gunkan-maki. So, for best texture, enjoy the sushi as soon as it is made.

Did you make this recipe? I’d love to see your recreations!

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9 months ago

Great recipe, not vegan though. Here in the US, most white white sugar is not vegan unlike Canada and most of the rest of the world.

Eric Horng
Eric Horng
9 months ago

This looks incredible! What brands would you recommend? I’m especially curious about the seaweed flakes, since I’ve never used them before.

9 months ago

Looks amazing Jorge!! I can’t wait to try this 🙂 I’ve never really liked raw fish before going vegan, but my bf and dad do so I will make this for them. Will update once I do! Thank you so much again, your posts always look so delicious!!!

9 months ago

Hey George! After roasting, how long can the softened watermelon keep in the fridge before you have to eat them? Kinda want to make them in large batches/meal prep style since the roasting time is a bit long.

9 months ago

For all those wondering: I made this last night and tried it just a moment ago! I ate it as a “sashimi” paired with some real tuna sashimi for some comparison. My notes: – I made 3 separate size comparisons (Nigiri “slices”, “sashimi block”, and cubes like in the recipe). – the marinade is not simply scalable for the weight of watermelon (I made ~5x the amount and had a lot of leftover marinade I saved for future cooks) – the marinating and cooking makes it quite salty for my tastes, so I actually decided to rinse& quick-soak the final… Read more »


[…] Prepare the Sushi Rice. Take some freshly steamed rice and place it into a bowl. Add the rice vinegar and sugar and mix well — do this while the rice is still hot so the sugar will melt. To shape the rice, dampen your hands with water and shape the rice into ovals. I also prepare sushi rice in a similar fashion in my Watermelon Sushi Recipe. […]