Rinse the scallops under running water and pat dry.
In a small bowl, prepare the cornstarch slurry (mix together the cornstarch and the 4 tbsps. water) and set aside.
In a separate bowl, whisk together the chicken broth, oyster sauce, rice wine or sherry or rice vinegar, and sugar to form the sauce; set aside.
Heat a wok or large skillet over medium-high to high heat; add the oil to the heated wok; when the oil is hot, add the garlic cloves and half the green onion; Stir-fry until the garlic is aromatic but not browned as that will give it a bitter edge.
Add the red bell pepper and the scallops and stir-fry for about 2 minutes, taking care not to overcook the scallops. Add the sauce and simmer the scallops and pepper in the sauce, uncovered, for about 5 minutes.
Give the cornstarch and water slurry a quick re-stir; add in the middle of the wok, stirring quickly to thicken; mix with the other ingredients.
Stir in the remainder of the green onion and garnish with the chopped cilantro.