Cube lamb leg into 1-centimeter (0.4-inch) cubes (or use ground lamb directly) and add them to a small bowl. Add 1 teaspoon cumin powder, 1 teaspoon vegetable oil, 1/2 teaspoon salt, the white pepper, and the cornstarch. Mix well and allow to marinate at room temperature for about 15 minutes.

While marinating the lamb, prepare the vegetables. Try to cut the onion and the potato the same size as the lamb cubes.

Add water to a large pot and bring to a boil (for cooking the noodles) . Prepare the sauce while boiling the water.

Heat 1 tablespoon oil in a large nonstick skillet (or a wok) over medium heat (medium high heat if using an electric stove) and swirl to coat the bottom of the wok with oil. When the oil becomes warm, add lamb. Stir and cook until the surface turns light brown but the meat is still raw inside, about 2 minutes. Transfer the lamb to a plate and set aside. Save the oil in the skillet.

Prepare a small bowl of water and set aside.

Add the remaining 2 teaspoons oil (or less) until the oil in the skillet is about 1 tablespoon. Add onion and stir and cook for 1 minute. Add potato and stir a few times. Swirl in 2 tablespoons water to speed up the cooking. Continue to stir and cook until the potato is slightly charred, 3 to 4 minutes. Add tomato, the remaining 1/2 teaspoon salt and the remaining 1 teaspoon cumin powder and mix a few times.

Add back lamb and stir to mix well. Add chili pepper (or bell pepper) and keep stirring until cooked through. Stop heat and transfer the sauce to a large bowl.

Cook noodles according to the instructions.

Add noodles to serving plates and pour the sauce on top. Serve immediately.