Heat 1 tbsp of oil in a skillet over medium-high heat. Add the broccoli florets and sauté for 4-5 minutes until tender-crisp. Remove and set aside.
In the same skillet, cook the ground beef and diced onion over medium-high heat. Crumble the beef as it cooks, then drain excess grease. Add the bell pepper and cook for 1 minute.
In a small bowl, mix soy sauce, oyster sauce, brown sugar, garlic, and beef broth. Pour it into the skillet with the beef mixture and bring to a gentle boil. Let it simmer for 2-3 minutes.
Shake the cornstarch slurry and slowly stir it into the sauce. Continue to bubble gently until the sauce thickens to your liking.
Reduce heat to low and stir the broccoli back into the skillet. Coat evenly with the sauce, then garnish with green onions. Serve hot with rice or noodles.