In a medium bowl, combine the tuna, avocado, and mayo. Mash until the tuna is flaked well, the avocado is mashed, and all of the ingredients are thoroughly combined. Add salt and pepper to taste, along with garlic powder and lime juice. Toss again.
Prepare a bowl with the rice and add the tuna mixture, cucumber, and coconut aminos. Stir to combine in the bowl.
Top with sesame seeds and green onion, and serve with seaweed and pickled ginger on the side! Eat the bowl with a fork, or scoop the mixture into the seaweed pieces and top with ginger. Roll and enjoy with your hands or chopsticks.