Prep Tomatoes: Score a small “X” in the bottom of each tomato. Drop into a pot of boiling water, removing after just 10 seconds, when skin begins to peel away. Immediately plunge tomatoes into a bowl of very cold water. The skin should come right off! Cut in half, remove seeds and insides, and cut into bite-sized pieces. Transfer to a bowl.
Marinade: Stir together soy, ginger, sesame oil, sriracha, and lime juice, then drizzle over the tomato. Toss to coat, and let marinate for at least 30 minutes.
Rice: Meanwhile, prepare your sushi rice. Add 1 cup cold water to the rice and set over high heat until water boils. Reduce heat to a gentle simmer and let cook for 15 minutes, covered. Remove from heat and let rest while you prepare the rest of the recipe. Stir together rice vinegar, sugar, and salt. Right before serving, stir this into the rice.
Assemble: Chop or thinly slice your fillings. Stir together mayonnaise and sriracha. Spoon rice into each serving bowl, topping with fillings and tomato tuna.