Make the tuna salad: Drain 2 (5-ounce) cans of oil packed tuna. Add the tuna to a medium bowl along with 5 tablespoons mayonnaise, 3 tablespoons sriracha, 2 tablespoon rice vinegar, 1 teaspoon sesame oil, and a big pinch each of salt and sugar. Using a fork, vigorously stir the mixture until it turns to a very pasty, well combined texture. There should not be large chunks of visible tuna, it should be homogenous. Taste and add more salt if needed. Cover with plastic wrap and chill until ready to assemble (at least 15 minutes).

Pickle the cukes: In a shallow bowl, stir together ¼ cup unseasoned rice vinegar, and two big pinches each of salt and sugar. Thinly slice 2 persian cucumbers lengthwise into ⅛” thick planks. Add the cukes to the pickling liquid and chill until ready to assemble.

Assemble: Arrange a rack in upper third of your oven. Preheat the broiler. Arrange 4 slices brioche on a rimmed baking sheet and toast, keeping an eye on things as they will brown very quickly, until golden brown on one side. Arrange the bread on a work surface, toasted sides up. Drizzle the bottom slices with mayo and the tops with some more sriracha. Slice two 1½” thick planks of iceberg from a whole head by cutting it crosswise (not through the root end). Very thinly slice 2 or 3 rings from a white onion. Place the lettuce planks on top of the mayo-d sides, followed by a layer of pickled cucumbers, a few rings of onion, half of the tuna salad and the second piece of toast, toasted side down. Cut in half. Sprinkle the cut sides of each sando with some toasted sesame seeds and serve.