Slice the tuna: Using a sharp knife, cut the tuna into 1-inch cubes. Place in a large bowl.
Combine all ingredients: Add the onions, garlic, sesame seeds, macadamia nuts, soy sauce, sesame oil, salt, and red pepper flakes. Gently mix until thoroughly combined.
Cover and refrigerate: Cover the poke with plastic wrap and refrigerate for a minimum of 2 hours or up to 2 days.