Heat cast iron skillet over medium-high heat and add oil or butter.
Sear steak in batches, 2–3 minutes on one side and 1 minute on the other. Remove and set aside.
Add remaining oil and vegetables. Stir-fry for 4–5 minutes until crisp-tender.
Combine soy sauce, oyster sauce, brown sugar, rice wine vinegar, garlic, ginger, cornstarch slurry, and sesame oil in a jar. Shake until smooth.
Add steak back to pan, pour sauce over, and toss to coat. Cook 2–3 minutes until sauce thickens.
Serve hot over jasmine rice.