Dissolve cornstarch in sherry, oyster sauce, and water. Add red pepper flakes and set aside.

Heat oil in a wok or large skillet over medium-high heat.

Add ginger and garlic and stir-fry for 1 minute until fragrant.

Add broccoli and stir-fry for 3 minutes.

Add green pepper, celery, and green onions, and stir-fry for another 3 minutes.

Create a well in the center of the wok and pour in the cornstarch mixture.

Stir continuously until the sauce thickens.

Add the sliced cooked beef and gently stir to coat.

If the mixture becomes too thick, add a splash of chicken broth to thin it out.

Serve immediately with steamed rice.