In a mixing bowl, add all the marinade ingredients and combine well.
Next, add the lamb and mix everything. Keep it aside. Marinate the lamb for a minimum of 1 hour.
Take a wok and add the cooking oil. Heat it over medium to high heat.
Now add cornstarch and gently mix everything. Make sure that the lamb is well-coated.
Once the oil is heated, add the first batch of marinated lamb and cook it from both sides until it develops a nice golden brown colour. Once cooked, keep it on a plate and set aside. Repeat the same with the next batch.
Take another mixing bowl and add all the sauce ingredients mentioned above. Give it a nice stir.
Next, pour in the remaining cooking oil and heat it over low to medium heat.
Now add the onions, green onions, ginger & garlic and stir fry for about 3 to 5 minutes. Cook them until they begin to soften and turn slightly brown in colour.
Add the cooked lamb into the wok and give a quick toss to make sure everything is mixed properly.
It's time to add the sauce and toss again, to make sure everything is well coated in the sauce.
Garnish it with green onions and sesame seeds.
Turn off the heat, and your delicious Mongolian lamb is ready to be served with steamed white rice!