01 - Bring a medium pot of water to a rolling boil. Add the udon noodles and cook according to package directions until tender but still chewy. Drain through a colander and rinse briefly under cold running water to halt the cooking process. Set aside and let cool completely.
02 - In a large mixing bowl, combine the drained tuna, mayonnaise, Sriracha, soy sauce, sesame oil, rice vinegar, and sugar. Stir thoroughly until all ingredients are evenly incorporated and a cohesive, saucy mixture forms.
03 - Add the julienned carrot, thinly sliced cucumber, spring onions, and pickled ginger to the tuna sauce. Fold gently with a spatula to combine without breaking apart the vegetables.
04 - Add the cooled udon noodles to the bowl with the spicy tuna and vegetable mixture. Toss vigorously using tongs or chopsticks, 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 add fresh chili slices if desired for extra heat.
06 - Serve immediately while the noodles are still cool, or let sit at room temperature for up to 20 minutes before enjoying. Pairs well with a light, dry sake or a crisp white wine.