Prep the sauce: In a small bowl, whisk soy sauce, oyster sauce, rice vinegar, honey, sesame oil, and water.
If you want a thicker sauce, stir in the cornstarch until smooth. Add chili flakes if you like heat. Set aside.
Prep the veggies and tuna: Drain canned tuna really well and flake lightly with a fork.
If using fresh tuna, pat dry and season with a pinch of salt and pepper.
Heat the pan: Set a large skillet or wok over medium-high heat. Add 1 tablespoon oil and swirl to coat.
Cook the aromatics: Add garlic, ginger, and the white parts of the green onions. Stir-fry for 30 seconds until fragrant, being careful not to burn.
Stir-fry the hard veggies: Add broccoli and carrots.
Cook 2–3 minutes, stirring often, until they start to soften but still feel crisp.
Add the quick-cook veggies: Add bell pepper and snow peas. Stir-fry 2–3 minutes more. You want bright color and a little bite.
Add the tuna: If using fresh tuna, push veggies to the edges, add the remaining 1 tablespoon oil, and sear tuna pieces 1–2 minutes per side until just cooked.
If using canned tuna, gently fold it in now.
Sauce it up: Give the sauce a quick stir and pour it over everything. Toss gently to coat and cook 1–2 minutes until the sauce thickens slightly and clings to the tuna and veggies.
Finish and serve: Sprinkle in the green parts of the onions and sesame seeds. Taste and adjust with a splash more soy sauce, vinegar, or chili.
Serve hot over rice or noodles.