Tekka Don (Japanese Tuna Sashimi Bowl)

Name: Tekka Don (Japanese Tuna Sashimi Bowl)
Prep time: 40 min
Cook time: 30 min
Total time: 1 h 10 min
Cuisine: Japanese
Ingredients: 150 g uncooked Japanese rice; 150 ml cold water; 3 g dried kelp (kombu) (kombu); ½ tbsp sake; 1 tbsp rice vinegar; ¼ tsp sugar; ½ tsp salt; 200 g sashimi grade tuna (sliced); 50 ml Japanese soy sauce (koikuchi shoyu); 50 ml mirin; 50 ml sake; 2-3 g dried kelp (kombu); 1 tsp wasabi paste; 1 tbsp pickled sushi ginger (gari); 2-4 perilla leaves (shiso); 2 tbsp finely chopped green onions