Rinse 2 cups of sticky rice under cold water until the water runs clear to remove excess starch. Combine the rice with 4 cups of water in a medium saucepan and bring it to a boil over high heat. Once boiling, reduce heat to low, cover, and let it simmer for 15 minutes. Turn off the heat and let it sit, covered, for another 10 minutes to finish steaming. Fluff the rice with a fork before serving.
Heat 2 tablespoons of vegetable oil in a large skillet or wok over medium-high heat. Add the minced garlic and grated ginger, stirring frequently for about 30 seconds until fragrant but not browned.
Add the thinly sliced chicken breast strips to the skillet. Stir-fry, tossing frequently, until the chicken is cooked through and no longer pink, about 5-7 minutes.
In a small bowl, whisk together 1/4 cup soy sauce, 2 tablespoons hoisin sauce, 1 tablespoon brown sugar, 1/2 teaspoon black pepper, and 1/4 teaspoon red pepper flakes if using. Pour the sauce over the cooked chicken and stir well to coat. Let it simmer for 2-3 minutes until the sauce thickens slightly and clings to the chicken.
Remove the skillet from heat and sprinkle the sliced green onions over the chicken. Serve the Mongolian Chicken hot alongside the fluffy sticky rice. The rice will soak up the savory sauce, making each bite irresistible.