1. Prepare Ingredients: Start by prepping all your ingredients. Cut the chicken breasts into bite-sized pieces. Slice the onion and bell peppers thinly, mince the garlic, and chop the green onions. Set everything aside, as the cooking process moves quickly.

2. Heat the Oil: In a large skillet or wok, heat the vegetable oil over medium-high heat. You want the oil hot enough so the chicken sears quickly and develops a nice golden brown color.

3. Cook the Chicken: Once the oil is heated, add the chicken pieces in a single layer. Allow them to cook for 5-6 minutes, stirring occasionally until the chicken is browned and cooked through. Remove the chicken from the skillet and set aside on a plate.

4. Stir-Fry Vegetables: In the same skillet, add the sliced onions and bell peppers. Stir-fry for about 3-4 minutes until the vegetables begin to soften but still retain some crispness. This step adds texture and color to the dish.

5. Add Garlic: Stir in the minced garlic and cook for 1 minute, stirring constantly. Garlic cooks quickly, so keep an eye on it to prevent burning. The aroma of the garlic should be fragrant.

6. Make the Sauce: Return the cooked chicken to the skillet with the vegetables. Pour in the soy sauce, brown sugar, and hoisin sauce, stirring to combine all the flavors. The combination of soy sauce and brown sugar gives the dish its signature savory-sweet taste, while the hoisin sauce adds depth and richness.

7. Thicken the Sauce: Once the sauce ingredients are well-mixed, add the cornstarch slurry (cornstarch mixed with water) to the skillet. Stir continuously until the sauce thickens, coating the chicken and vegetables evenly. This will only take about 1-2 minutes.

8. Garnish and Serve: Finally, stir in the chopped green onions for a burst of freshness. Serve the Mongolian chicken hot over a bed of cooked rice.