Heat the oil & butter in a skillet over moderate heat. When the butter has melted, add in one layer of chicken.
Insert a pepper & salt to chicken. Let me cook for 3 mins, or until brown then turn onto the opposing side. Let cook another 3 mins, or till golden-brown. Turn the heat to reduce and then stir in the garlic and cook.
While the chicken is cooking, whisk together the chicken stock, soy-sauce, hoisin-sauce, vinegar, and wheat starch in a little bowl. Stir in the skillet and cook chicken over low heat, until the sauce has thickened. Stir in cashews.
Serve in the lettuce cups over rice if chosen. Top with chopped green-onions if desired.