1 slab of raw sushi-grade tuna OR 1 package (about 1 cup) of preground raw tuna belly - sometimes Japanese markets have these. If you can't buy sushi-grade tuna near you, order it from Catalina Offshore Products.
2 scallions, finely chopped
2 tablespoons mayonnaise (use Kewpie if you can find it, if not, regular mayo is fine)
Chili oil or Sriracha sauce to taste - add 1/4 tsp at a time until you reach your desired heat level
Dice the tuna in small pieces and then chop it as finely as you can. Place it in a bowl and mix in the scallions, mayonnaise, chili oil or sriracha, and mix well. And that's it! From here you can serve it over rice and have a spicy tuna bowl, or roll it into maki sushi rolls or hand rolls. You can even just make the spicy mayonnaise on its own by combining everything above except the fish, and add the mayo to other fish such as salmon, yellowtail, scallops, mussels ... remember, the spicy mayo is the key ingredient in many sushi items called "Dynamite" and "Volcano." Sushi chefs put the spicy mayo in a squeeze bottle and decorate dishes with fancy swirls of it. Try searing some ahi tuna and squeezing a few lines of spicy mayo around it.
(Image: Kathryn Hill)