In a medium bowl, combine the thinly sliced beef with 1 tablespoon of soy sauce and 1 tablespoon of cornstarch. Toss well to coat every piece. Set aside.

In a small bowl, whisk together the remaining soy sauce, oyster sauce, brown sugar, sesame oil, and the remaining cornstarch. Set aside.

Heat a large wok or skillet over high heat. Add 1 tablespoon of oil. Once shimmering, add the beef in a single layer (work in batches if necessary). Sear for 1-2 minutes per side until browned. Remove from skillet and set aside.

Add the remaining oil to the skillet. Add the garlic and ginger and cook for 30 seconds until fragrant. Add the white parts of the bok choy and stir-fry for 2-3 minutes. Add the green leafy parts and cook for another minute until wilted.

Return the beef to the skillet. Whisk the sauce again and pour it into the pan. Stir continuously for 1-2 minutes until the sauce thickens and coats everything. Serve immediately over hot rice.