Place the cubed beef in a zip-top bag with flour, cornstarch, white pepper, and salt. Shake until well-coated.

Heat oil in a large skillet or wok over medium-high heat. Add the coated beef cubes in batches, cooking until they are browned on all sides. Remove and set aside.

In the same skillet, add broccoli, carrots, onion, and bell peppers, cooking until they begin to soften. Add zucchini and cook for an additional few minutes.

In a bowl, combine soy sauce, hot water, and minced garlic, then pour the mixture into the skillet with the vegetables.

Return the cooked beef to the skillet, tossing to coat with the sauce. Let simmer until everything is heated through, adjusting seasoning as needed. Serve hot over rice if desired.