Remove and discard the tendon from the turkey; dry the turkey breast with paper towels. Dice the turkey into ½-inch pieces.
In a medium bowl, combine the salt, sugar, pepper, and cornstarch. Add the turkey, cover, and refrigerate for 20 minutes.
Carefully remove the lettuce leaves from the head. Take the 6 largest, nicest bowl-shaped leaves. Reserve the remaining leaves for another use. Trim the edges to form a 4-inch bowl.
Heat a large, deep nonstick skillet with a high edge over high heat. Add the vegetable oil, turkey, water chestnuts, and shallots. Using 2 spoons, stir and separate until the turkey turns white, about 2 minutes. Add the pimiento and stir-fry for 1 minute. Remove from the heat.
Brush approximately 1 teaspoon hoisin sauce, or to taste, on each lettuce leaf. Place 1/3 cup filling in the center of each lettuce leaf, garnish with green onion, and serve with the remaining hoisin sauce.