Mix the soy sauce, oyster sauce, and cornstarch in a bowl until smooth. Add the thinly sliced beef and toss to coat. Let it marinate for about 15 minutes while you prep vegetables.

Heat 1 tablespoon vegetable oil in a large skillet or wok over medium-high heat until shimmering. Shake off excess marinade and add beef in a single layer. Stir-fry just until browned, about 1–2 minutes. Remove beef to a plate.

Add the remaining tablespoon of oil to the same skillet. Add minced garlic and ginger and stir for 30 seconds until fragrant—don’t let them burn.

Add mixed vegetables and stir-fry, tossing constantly, until they are bright and tender-crisp, about 3–5 minutes depending on size.

Return the beef to the skillet. Pour any remaining sauce from the bowl over the pan and stir everything together for 2–3 minutes. The cornstarch will thicken the sauce; cook until it’s glossy and coats the meat and vegetables.

Taste and season with salt and pepper as needed. Serve right away over rice or noodles.