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 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 sauce is smooth and cohesive.
03 - Add the julienned carrot, sliced cucumber, spring onions, and pickled ginger to the spicy tuna sauce. Fold gently with a spatula to distribute the 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 two forks, ensuring every strand is thoroughly coated with the sauce.
05 - Divide the dressed noodles evenly between two serving bowls. Sprinkle generously with toasted sesame seeds, scatter the torn nori pieces over the top, and arrange fresh chili slices if desired.
06 - Serve immediately, either chilled or at room temperature. Pairs excellently with a light, dry sake or a crisp white wine.