Drain 2 (6-ounce) cans tuna and place in a large bowl. Break up any large chunks of tuna with a fork.

Prepare the following, adding them to the bowl as they are completed: Finely chop 1 small red onion (about ¾ cup). Finely chop 2 stalks celery (about 1 cup). Coarsely chop fresh parsley leaves until you have ¼ cup. Finely grate the zest of 1 medium lemon until you have 1 teaspoon zest (reserve the lemon).

Halve and pit 1 medium avocado. Cut one half of the avocado into ⅓- to ¼-inch pieces. Add to the bowl with tuna and gently mix until well-combined, doing your best to keep the avocado pieces intact.

Prepare the following, adding them to a medium bowl as you complete them: Scoop out the flesh from the remaining avocado half, finely grate 1 small garlic clove, juice the lemon until you have 1 tablespoon juice, use a whisk to mash the avocado, then whisk until smooth.

Finely chop fresh chives until you have 2 tablespoons. Add the chives, 3 tablespoons extra-virgin olive oil, and ½ teaspoon kosher salt to the smooth avocado. Whisk until smooth and well-combined. Transfer to the bowl with the tuna and gently mix until everything is evenly coated in the mixture. Taste and season with more lemon juice and kosher salt as needed. Serve as is or on the bread of your choice.