In a small bowl, combine the soy sauce, toasted sesame oil, rice vinegar, and 1 tsp sriracha. Add the sushi-grade tuna cubes and the thinly sliced scallions, stirring gently to coat evenly without breaking up the delicate fish. Let this mixture marinate while you prepare the remaining components—the acid from the vinegar and soy sauce will lightly cure the tuna and develop its flavors.

While the tuna marinates, prepare the spicy mayo by whisking together the mayonnaise and 2 1/2 tsp sriracha in a small bowl until smooth and evenly colored. In parallel, slice the cucumber thinly, cut the avocado in half, remove the pit, and slice the flesh thinly (I like to slice the avocado just before assembly to prevent browning, but if you're concerned about oxidation, a light squeeze of lemon or lime juice works wonders). Have the cooked rice, sliced scallions, sesame seeds, and furikake ready at your assembly station.

Divide the warm cooked rice between two bowls, creating a base for each. Arrange the marinated tuna from Step 1 on top of the rice, then add the sliced cucumber and avocado in separate sections around the bowl—this creates visual appeal and lets each component shine. Drizzle the spicy mayo from Step 2 generously over the tuna, then sprinkle the sesame seeds and furikake (if using) on top, finishing with the sliced scallions as a fresh garnish.

Serve the bowls immediately while the rice is still warm and the avocado is at its best. Offer extra soy sauce and sriracha on the side for guests to adjust the seasoning to their preference—this is especially nice since everyone's heat tolerance differs.