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, then 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 and rinse briefly under cold water to halt cooking, then set aside.
03 - In a separate saucepan, combine the dashi broth, soy sauce, mirin, and sugar. Bring to a gentle simmer over medium heat, stirring until the sugar fully dissolves. Remove from heat once the broth is fragrant and well blended.
04 - Divide the cooked udon noodles evenly between two serving bowls. Ladle the hot dashi broth over the noodles, ensuring they are well submerged.
05 - Spoon the marinated spicy tuna generously over the noodles and broth in each bowl, distributing it evenly.
06 - Finish each bowl with sliced green onions, shredded nori, toasted sesame seeds, and julienned cucumber. Drizzle with chili oil and add pickled ginger if desired. Serve immediately while hot.