Marinate the Pork: Place the sliced pork tenderloin in a mixing bowl. Add 2 tablespoons of soy sauce and 1/4 teaspoon of black pepper. Toss the pork until it's evenly coated with the soy sauce and pepper.

Coat the Pork in Cornstarch: Sprinkle 3 tablespoons of cornstarch over the marinated pork. Toss the pork slices until each piece is lightly coated in cornstarch. This will help the pork develop a crispy exterior when fried.

Fry the Pork: In a large skillet or wok, heat 2 tablespoons of cooking oil over medium-high heat. Once the oil is hot, add the pork in batches to avoid overcrowding. Fry the pork strips for 2-3 minutes per side until golden and crispy. Remove the pork from the pan and set aside on a plate.

Sauté the Aromatics: In the same pan, add a little more oil if necessary. Add the minced garlic, minced ginger, and sliced onion. Sauté for 2-3 minutes until the onions are translucent and the garlic and ginger are fragrant.

Add the Vegetables: Add the sliced red bell pepper to the pan and stir-fry for another 2-3 minutes. The vegetables should be cooked but still crisp, retaining their vibrant colors.

Combine the Sauce Ingredients: In a small bowl, whisk together 1 tablespoon of soy sauce, 1 tablespoon of oyster sauce, 1 tablespoon of chili-garlic sauce, 1 tablespoon of brown sugar, and 1/2 tablespoon of sesame oil. Set this sauce mixture aside.

Make the Cornstarch Slurry: In a separate bowl, mix 2 teaspoons of cornstarch with 2 tablespoons of water to make a slurry. This will help thicken the sauce later.

Add the Pork Back to the Pan: Return the cooked pork strips to the pan with the sautéed vegetables. Toss everything together so the pork and vegetables are evenly mixed.

Pour in the Mongolian Sauce: Pour the prepared Mongolian sauce over the pork and vegetables. Stir well to coat everything in the sauce.

Add Water and Thicken: Pour 1 cup of water into the pan and bring the mixture to a simmer. Gradually stir in the cornstarch slurry and cook for another 2-3 minutes, or until the sauce thickens to your desired consistency.

Add Spring Onions: Once the sauce has thickened, sprinkle chopped spring onions over the top and give the dish a final stir.

Serve Hot: Transfer the Mongolian Pork Stir-Fry to a serving dish. Garnish with additional spring onions if desired.

Pair with Sides: Serve with steamed rice or noodles for a complete meal. The rich, flavorful sauce pairs perfectly with plain rice, soaking up all the savory goodness.