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 further cooking. 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 the mixture forms a cohesive, creamy sauce.
03 - Add the julienned carrot, thinly sliced cucumber, spring onions, and pickled ginger (if using) to the spicy tuna sauce. Fold gently with a spatula to coat all vegetables evenly without breaking them down.
04 - Transfer the cooled udon noodles into the bowl with the spicy tuna and vegetable mixture. Toss vigorously using tongs or chopsticks, lifting and turning the noodles until every strand is well coated with the sauce.
05 - Divide the dressed noodles evenly between two serving bowls. Sprinkle generously with toasted sesame seeds, scatter torn nori pieces over the top, and arrange fresh chili slices if desired for additional heat.
06 - Serve immediately, either chilled or at room temperature, for the best texture and flavor balance. Pairs wonderfully with a light, dry sake or a crisp white wine.