Cook the brown rice according to package instructions and set aside to cool slightly.
Using a sharp knife, cut the sushi-grade ahi tuna into uniform 1/2-inch cubes.
In a small glass bowl, whisk together the coconut aminos, sesame oil, lime juice, and red pepper flakes to create the marinade.
Place the tuna cubes in a medium bowl and pour the marinade over them, tossing gently to coat every piece.
Let the tuna marinate in the refrigerator for 10 minutes to allow the flavors to penetrate.
Prepare the tropical salsa by combining the diced mango, minced red onion, and chopped cilantro in a small dish.
Assemble the bowl by layering the cooked brown rice at the base, then topping with the marinated tuna.
Arrange the sliced cucumber, avocado, and mango salsa around the tuna.
Finish the dish by sprinkling sesame seeds and sea salt over the top for a final touch of texture and flavor.