Cut raw tuna steaks into 1-in. thick cubes.
Add tuna, cucumber, avocado, green onion, pepper and cilantro to a mixing bowl. Stir gently to combine.
In a small bowl, mix garlic, ginger, sesame oil, coconut aminos, red pepper flakes, fish sauce and salt. Add to the large mixing bowl and stir to combine again.
Mix in sesame seeds, seaweed flakes, and more salt if desired.
Place in fridge and allow to marinate, covered, for 2 hours or more.
Garnish with lime and enjoy within 2 days.