In a small bowl, combine the sauce ingredients. Whisk well and set aside.
Heat a wok or large frying pan over medium-high heat.
Swirl in oil to coat all sides of the wok. When the oil is hot, add in garlic, ginger, and the white scallion pieces.
Stir-fry for just a few seconds, or until aromatic.
Add in the chopped chicken pieces, and stir-fry over high heat until almost cooked though.
Add in the bell peppers and continue to cook for another minute.
Add in the sauce. Give everything a few tosses to combine.
Stir-fry until the sauce has cooked down and coated everything nicely.
Remove from heat. Have a quick taste and adjust any seasonings if desired.
Add in the diced cucumber, and toss again to combine. Dish out in a serving bowl.
On the table, place out separate dishes of lettuce leaves, the stir-fried chicken & vegetables, chopped cashews, julienned carrots, and the green scallion pieces. Then have the family or your party guests make their own lettuce cups.