Serves 8 to 10 as an appetizer. Serve with restaurant style tortilla chips.
Freeze the tuna until semi-firm, about 40 minutes, to make it easier to dice. Cut the frozen tuna steaks into ¼-inch cubes and place in a non-reactive serving bowl. Add the lime juice, cilantro, shallot, rice vinegar, and chile, and salt. Stir gently to combine, cover with plastic wrap, and let the fish marinate in the refrigerator for 30 minutes. Do not marinate longer than 30 minutes or the fish will become mushy and “over-cooked.” If you’d like to make the dish ahead, prepare the tuna and keep it separate from the other ingredients until ready to mix and marinate.
Drain any accumulated juices in the bottom of the serving bowl and discard.
Halve the avocados lengthwise, remove the pit, and cut a fine crosshatch pattern into the avocado flesh without cutting into the peel. Scoop the diced avocado flesh out and add it to the bowl with the remaining ingredients, tossing gently with a spoon. Taste the ceviche on a crisp or tortilla chip and season with salt, pepper, and togarashi to taste. Arrange the radish slices on top of the ceviche around the edge of the bowl. Serve immediately, or cover with plastic and refrigerate for up to 30 minutes.