Combine half the cornstarch (reserve remaining for sauce), half the ginger (reserve remaining for cooking beef), half the sherry (reserve remaining for sauce), and sugar in a medium mixing bowl. Rinse beef strips, pat dry, and add to bowl, stirring to coat completely. Marinate at least 10 minutes. While meat marinates, prepare ingredients.
Prepare the Ingredients
Trim ¼" off ends of Chinese broccoli and separate stems from leafy tops. Cut stems into 2" pieces. Coarsely chop tops. Mince garlic. Combine remaining cornstarch, oyster sauce, ¼ cup water, and remaining sherry in a small bowl and set aside.
Cook the Rice
Bring a small pot with rice and 1 ½ cups water to a boil. Reduce to a simmer, cover, and cook until rice is tender, 18-20 minutes. Remove from burner, keep covered, and set aside. While rice cooks, begin vegetables.
Cook the Vegetables
Heat a large non-stick pan over medium-high heat. Add 2 tsp. olive oil, ½ cup water, and broccoli stems to hot pan and cook, stirring occasionally, until water has evaporated and broccoli is crisp tender, 4-6 minutes. Add broccoli tops and cook, stirring constantly, until wilted, 1-2 minutes. Season with ¼ tsp. salt and remove to a plate. Wipe pan clean and reserve.
Cook the Beef
Return pan used to cook vegetables to high heat and add 2 tsp. olive oil, garlic, remaining ginger, and dried chiles (to taste- 1 chile is spicy and 4 is very spicy) to very hot pan and cook until golden brown and fragrant, 30-60 seconds. Add sliced beef to pan and cook, stirring constantly, until browned, 2-4 minutes. Stir cornstarch mixture to recombine and add Chinese broccoli and cornstarch-oyster sauce mixture to pan and cook until sauce is bubbling and thickened, 2-3 minutes.
Plate the Dish
Spoon stir-fry on a plate and serve rice alongside.