In a pan, heat olive or canola oil. Put onions and garlic, add the beef strips. Stir occasionally and simmer for 3 minutes. Put water and boil for 10 minutes, season with salt, pepper and soy sauce. Add the cornstarch and stir. Boil for another minutes until sauce thickens. Add mushrooms and asparagus and cook not more than 5 minutes to preserve the green color and crispness of the asparagus. Garnished with leeks. Serve hot.