01 - In a mixing bowl, combine the diced sushi-grade tuna with mayonnaise, Sriracha, soy sauce, sesame oil, and rice vinegar. Toss gently until the tuna is evenly coated. Cover and refrigerate for at least 10 minutes to allow the flavors to meld.
02 - Bring a large pot of water to a rolling boil. Add the udon noodles and cook according to package directions until tender but still chewy. Drain in a colander and rinse briefly under cold water to halt cooking. Set aside.
03 - In a separate saucepan, combine the dashi broth, soy sauce, mirin, and sugar. Place over medium heat and bring to a gentle simmer, stirring until the sugar dissolves completely. Remove from heat once the broth is fragrant and well blended.
04 - Divide the drained udon noodles evenly between two serving bowls. Ladle the hot dashi broth over the noodles, ensuring each bowl receives an equal portion.
05 - Spoon the marinated spicy tuna generously over the noodles and broth in each bowl, distributing it evenly across the surface.
06 - Finish each bowl with sliced green onions, shredded nori, toasted sesame seeds, julienned cucumber, and any optional toppings such as chili oil or pickled ginger. Serve immediately while hot.