Optional — Soak the minced red onion in an ice water bath with a palmful of fine sea salt while continuing the prep. It removes some of the “bite” of the onion!
Make the sauce — Whisk together the juice and zest of the orange, the ají amarillo paste, your choice of ponzu, tamari or soy sauce, and sesame oil in a bowl large enough to hold both the ahi tuna and the onion.
Add the tuna to the sauce — After dicing the tuna, combine it with the sauce. Drain the water from the onion, and add it to the tuna along with the chopped cilantro. (Note: If any ice remains with the onion, put the onion in a strainer, and run under cold water before adding). Stir well to combine.
To serve — In a small bowl (or martini glass!), add diced avocado, then spoon the tuna mixture over top. Make sure you include plenty of sauce, as it will moisten the avocado as well. Garnish as desired, and enjoy!