Combine soy sauce, mirin, sesame seeds, shallot, garlic, ginger, sesame oil, red pepper flakes, and sriracha in a medium bowl; whisk until well combined.
Add tuna and avocado; stir gently to coat with marinade.
Place tuna in a medium non-reactive bowl. Add soy sauce, green onions, sesame oil, sesame seeds, macadamia nuts, and red pepper flakes; mix well. Cover and refrigerate at least 2 hours before serving.
Whisk scallion, soy sauce, chili-garlic sauce, sesame oil, rice vinegar, and red pepper flakes in a bowl until thoroughly combined. Add tuna cubes and toss until well coated. Sprinkle sesame seeds on top.