Preheat oven to 375°F (190°C). Grease or line an 8×8-inch baking dish.

Rinse the sushi rice under cold water until clear. Combine rice and water in a pot, bring to a boil, then reduce to low, cover, and simmer for 18 minutes. Remove from heat and let stand, covered, 10 minutes.

In a small bowl, whisk rice vinegar, sugar, salt, and 1 tablespoon mayo. Gently fold into the cooked rice until well coated.

Arrange the vinegared rice evenly in the bottom of the baking dish, pressing down gently.

In another bowl, mix tuna (or other seafood) with 2 tablespoons mayo and Sriracha until creamy and slightly spicy.

Spread the seafood mixture evenly atop the rice layer.

Sprinkle the shredded cheese over the seafood.

Bake for 12–15 minutes, until the cheese is melted and bubbly.

Switch to broil for 1–2 minutes if you’d like the top golden and slightly crisp.

Remove from oven, top with nori pieces, green onions, sesame seeds, and pickled ginger.

Serve by scooping with extra nori sheets, cucumber slices, or enjoy straight from the dish.