
Place beef in a mixing bowl. Add 1 teaspoon minced garlic, ¼ teaspoon salt, ⅛ teaspoon black pepper, cornstarch and water. Toss to combine, then mix in 1½ teaspoons cooking oil. Set aside to marinate for 10-15 minutes.
Place a large pot filled with water over medium-high heat. Bring it to a boil, and then add the Tonkin flowers. Blanch the flowers for a minute or slightly more. Remove them from the pot and give them a rinse under cold water. Set aside to drain.
Place a heavy-bottomed pan over medium-high heat. Once the pan is hot, add cooking oil to coat the bottom. Add the beef in a single layer and let them cook undisturbed briefly for about 20-30 seconds. Then add the oyster sauce, and stir-fry for a few more seconds and transfer to a plate while the beef is still slightly pink.
Add more cooking oil to the pan if needed. Adjust the heat to medium and add the remaining garlic. Sauté briefly until the garlic is fragrant.
Add the blanched Tonkin flowers. Stir and toss to cook for about 1-2 minutes until the doneness is almost to your liking. Season with ⅛ teaspoon salt.
Return the beef to the pan. Stir and toss until both the beef and flowers are cooked through. Add the fish sauce (or soy sauce) and give everything a final stir. Adjust seasonings to taste.
Transfer to a serving plate and sprinkle with more black pepper before serving.