Step 1: Velvet the Chicken
In a medium bowl, combine the bite-sized chicken pieces with 1/4 cup of cornstarch, 1 tablespoon of soy sauce, and 1 tablespoon of vegetable oil. Use your hands or a spoon to toss everything together until every piece of chicken is evenly and thoroughly coated in a thick, pasty layer. The cornstarch coating should not be dusty or dry. Set this aside to marinate for at least 15-20 minutes while you prepare the other ingredients.

Step 2: Prepare the Mongolian Sauce
In a separate small bowl or liquid measuring cup, whisk together the sauce ingredients: 1/2 cup low-sodium soy sauce, 1/2 cup water (or broth), and 1/2 cup packed dark brown sugar. Continue whisking until the brown sugar is completely dissolved. Stir in the 1 tablespoon of minced ginger and 3 cloves of minced garlic. Set this sauce mixture aside. In another, even smaller bowl, prepare your cornstarch slurry by whisking together 1 tablespoon of cornstarch and 2 tablespoons of cold water until smooth. Keep this slurry separate; it will be used to thicken the sauce at the very end.

Step 3: Prepare the Aromatics and Set Up Your Station
Wash and chop your scallions, separating the white/light green parts from the dark green parts. Mince the additional garlic and ginger for the stir-fry. Place all your components—the coated chicken, the sauce mixture, the cornstarch slurry, and the aromatics—near your stove. A stir-fry moves fast, so having everything within reach is essential.

Step 4: Cook the Chicken
Heat 1/4 cup of oil in a large wok or a heavy-bottomed skillet over medium-high heat. The oil should be hot enough that a piece of chicken sizzles immediately upon contact. Carefully add half of the coated chicken to the hot oil in a single layer, making sure not to overcrowd the pan. Fry for 2-3 minutes per side, until the chicken is golden brown, crispy, and cooked through. Use a slotted spoon to remove the cooked chicken and place it on a wire rack or a paper-towel-lined plate. Repeat with the second batch of chicken.

Step 5: Build the Flavor Base
Pour out all but 1 tablespoon of the oil from the wok. Reduce the heat to medium. Add the white and light green parts of the scallions, along with the additional minced garlic and ginger. Stir-fry for about 30-60 seconds until fragrant and aromatic. Be very careful not to let the garlic burn.

Step 6: Create the Sauce
Pour the prepared soy sauce and brown sugar mixture into the wok with the aromatics. Bring the sauce to a vigorous simmer, stirring occasionally. Let it bubble for 1-2 minutes to allow the flavors to meld.

Step 7: Thicken the Sauce
Give your cornstarch slurry a quick re-whisk to ensure it’s smooth. While constantly stirring the simmering sauce in the wok, slowly pour in the slurry. Continue to stir, and within 30-60 seconds, you will see the sauce magically transform. It will thicken, become glossy, and beautifully coat the back of a spoon.

Step 8: Combine and Finish
Turn off the heat. Immediately add the crispy cooked chicken and the dark green parts of the scallions to the wok. Toss everything together quickly but gently, ensuring every piece of chicken is enveloped in the luscious, glossy sauce. The goal is to coat, not to cook further, which preserves the chicken’s crispy texture.

Step 9: Serve Immediately
Transfer the Mongolian Chicken to a serving platter. Garnish with sesame seeds if desired, and serve immediately with hot steamed rice.