Core and cut ¼ medium green cabbage into ½-inch wide shreds (about 2 cups). Thinly slice 1 serrano pepper crosswise if desired (about 1 tablespoon). Thinly slice 2 medium scallions (about ¼ cup); reserve about 1 tablespoon of the dark green parts for garnish. Thinly slice ½ medium red onion (about ¼ cup). Mince 3 garlic cloves (about 1 tablespoon).
Place 3 tablespoons soy sauce, 1 tablespoon honey, 1 tablespoon mirin, 2 teaspoons toasted sesame oil, 1 teaspoon rice vinegar, 1 teaspoon oyster sauce, and 1 teaspoon coarsely ground black pepper in a small bowl and stir to combine.
Open 2 (5-ounce) cans albacore solid tuna in extra-virgin olive oil. Drain the oil from the canned tuna into a large nonstick or a cast iron skillet. Heat over medium-high heat until shimmering. Add the garlic and remaining scallions, and cook until fragrant, about 1 minute.
Add the cabbage, onion, and 1 tablespoon of the sauce. Cook until the cabbage and onions are slightly translucent and tender, 3 to 4 minutes. Add the tuna, remaining sauce, and serrano if using. Stir-fry until combined and the tuna is all mixed through the cabbage, 4 to 5 minutes.
Garnish with the reserved scallions and 1 teaspoon toasted white sesame seeds. Serve over white rice.
