In a bowl, toss the chicken with about a third of the minced garlic, the white pepper, soy sauce, oyster sauce, fish sauce, sugar and cornstarch. Mix until every piece is coated and let it sit for 15 minutes while you prep everything else. The cornstarch is doing double duty here, it helps the chicken brown and it thickens the pan sauce at the end.
Heat 1 tbsp of the oil in a wok or large skillet over medium heat. Add half of the remaining garlic and fry it slowly, stirring, until it turns pale gold and crisp, about 2 minutes. Watch it closely, garlic goes from golden to burnt in seconds. Scoop the crispy garlic out onto a plate and set it aside for the top.
Turn the heat to medium-high and add the last 2 tbsp of oil. Add the chicken in a single layer and let it sear undisturbed for 2 to 3 minutes so it picks up color before you stir. Then toss and cook for another 4 to 5 minutes until the pieces are cooked through and browned in spots.
Add the last of the raw minced garlic and stir for 30 seconds until fragrant. Pour in the water or broth, scraping up the browned bits stuck to the pan. Let it bubble for a minute until the sauce thickens and clings to the chicken.
Taste and adjust with a splash more fish sauce or a pinch more white pepper if you like it sharper. Pile onto a plate, scatter the reserved crispy garlic and the cilantro over the top, and serve with jasmine rice and cucumber slices.