In a 12-inch nonstick skillet over medium, combine 3 tablespoons oil, the garlic and scallion whites; cook, stirring, until the garlic is golden brown, about 1 minute. Add the chicken, soy sauce and vinegar. Cook, stirring occasionally, until the liquid has evaporated and the chicken is browned in spots, about 7 minutes.
Drizzle the chicken mixture with the remaining 1 teaspoon oil. Add the rice, ¼ teaspoon salt and 1 teaspoon pepper, breaking up any clumps. Increase to medium-high and cook, stirring occasionally, until the rice is sizzling and heated through, 3 to 4 minutes. Serve sprinkled with the scallion greens and additional pepper.
Optional garnish: Thinly sliced Fresno or jalapeño chilies
