Place the tuna in the refrigerator for at least 15 minutes to firm it up before slicing.
Remove the tuna and cut it into 1/4-inch dice. Return the cubes to a bowl and keep refrigerated.
Peel and dice the avocado into 1/4-inch pieces.
Finely chop the red onion.
In a separate bowl, whisk together the lime juice, lime zest, low-sodium soy sauce, freshly grated ginger, and toasted sesame oil.
Take the tuna from the fridge, add the avocado, chopped red onion, and red pepper flakes (if using). Pour in the dressing and gently toss to combine.
Taste and adjust with salt or more lime juice if needed.
Cover the bowl with plastic wrap and chill for 15–20 minutes to let the flavors meld.
For a neat presentation, use a ring mold to shape the tartare, then sprinkle with black sesame seeds before serving.