01 - Bring a medium pot of water to a rolling boil. Add the udon noodles and cook according to package instructions until tender but still chewy. Drain in a colander and rinse briefly under cold running water to halt the cooking process. Set aside to cool completely.
02 - In a mixing bowl, combine the drained tuna, mayonnaise, Sriracha, soy sauce, sesame oil, rice vinegar, and sugar. Stir thoroughly until all components are evenly incorporated into a cohesive, well-seasoned sauce.
03 - Add the julienned carrot, thinly sliced cucumber, spring onions, and pickled ginger to the tuna sauce mixture. Fold gently to combine without breaking apart the tuna or crushing the vegetables.
04 - Add the cooled udon noodles to the bowl with the spicy tuna and vegetable mixture. Toss vigorously, ensuring every strand is evenly coated with the sauce.
05 - Divide the dressed noodles between two serving bowls. Sprinkle generously with toasted sesame seeds, scatter torn nori pieces over the top, and arrange fresh chili slices if desired.
06 - Serve immediately, either chilled or at room temperature. Pair with a light, dry sake or crisp white wine for a complete dining experience.