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 slightly chewy. Drain in a colander and rinse briefly under cold water to halt the cooking process. Set aside to cool.
02 - In a mixing bowl, combine the drained tuna, mayonnaise, Sriracha, soy sauce, sesame oil, rice vinegar, and sugar. Stir thoroughly until all ingredients are well incorporated and the mixture forms a cohesive, spicy sauce.
03 - Add the julienned carrot, sliced cucumber, spring onions, and pickled ginger (if using) to the spicy tuna mixture. Fold gently to combine without breaking up the tuna too finely.
04 - Add the cooled udon noodles to the bowl with the spicy tuna and vegetable mixture. Toss vigorously using tongs or chopsticks until 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 additional heat is desired.
06 - Serve immediately, either chilled or at room temperature. Pairs well with a light, dry sake or a crisp white wine.