01 - Bring a medium pot of water to a rolling boil. Cook udon noodles according to package instructions until tender but still slightly chewy. Drain through a colander and rinse briefly under cold running water to halt further cooking. Set aside to drain completely.
02 - In a mixing bowl, combine the drained tuna, mayonnaise, Sriracha, soy sauce, sesame oil, rice vinegar, and sugar. Stir vigorously until all ingredients are evenly incorporated and a cohesive, creamy sauce forms.
03 - Add the julienned carrot, sliced cucumber, spring onions, and pickled ginger to the spicy tuna sauce. Fold gently with a spatula to coat the vegetables evenly without breaking them down.
04 - Transfer the cooled, well-drained udon noodles into the bowl with the spicy tuna and vegetable mixture. Toss thoroughly 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 arrange fresh chili slices if desired for added heat.
06 - Serve immediately, either chilled or at room temperature. Pairs well with a light, dry sake or a crisp white wine.