Heat olive oil in a large skillet or wok over high heat. Add beef and stir-fry until browned, about 2-3 minutes. Remove beef and set aside.
Add bell peppers and onion to the skillet and stir-fry until slightly softened, about 3-4 minutes. Add garlic and cracked black peppercorns, and cook for another minute until fragrant.
Pour in beef broth and soy sauce. Bring to a simmer and cook for 2 minutes.
Stir in cornstarch slurry and cook until the sauce thickens, about 1 minute.
Return beef to the skillet and stir to coat with the sauce. Cook for another minute until heated through.
Drizzle with sesame oil before serving. Serve hot over rice or noodles.
Notes
For a spicier version, add a pinch of red pepper flakes. You can also add other vegetables like mushrooms or broccoli.