Tekka Maki (Tuna Sushi Rolls)

Name: Tekka Maki (Tuna Sushi Rolls)
Prep time: 30 min
Total time: 30 min
Cuisine: Japanese
Ingredients: 1 tbsp rice vinegar; 1 tsp sugar; ½ tsp salt; 320 g cooked Japanese short-grain rice; ¼ tsp salt; 100 g sashimi-grade tuna; wasabi paste (optional); 2 sheets roasted seaweed for sushi (nori); 1 bowl cold water; 2 tbsp Japanese soy sauce (koikuchi shoyu) (for dipping)