Combine orange juice, soy sauce, rice vinegar, sugar, and cornstarch in a bowl.
Set aside.
Heat ½ cup of water in a large skillet until boiling.
Add broccolini and a pinch of salt.
Cover and steam for 2-3 minutes, then uncover and let the water evaporate.
Turn off the heat and set aside.
In a separate nonstick skillet, heat vegetable oil over medium-high heat.
Add the sliced beef and cook until crispy and brown.
This should take about 4-5 minutes.
Pour the orange juice mixture over the beef.
Stir well and let it simmer until the sauce thickens.
It should become glossy and cover the beef nicely.
Toss in the steamed broccolini.
Mix everything together for another minute so flavors blend.
Serve warm with your choice of rice or noodles.