Heat the olive oil in a large pan set over medium-high heat. Add the onion and cook until softened, about 5 minutes. Add the garlic and cook 1 minute.

Add the ground turkey and cook until browned, breaking the meat up with a wooden spoon as it cooks. Add the mushrooms, cole slaw mix, bell pepper, fish sauce, Thai seasoning, chili sauce (if using) and water chestnuts (if using). Stir until combined. Cook 10 minutes, or until the vegetables are cooked through. Remove from heat and stir in the lime juice.

Serve the turkey filling in lettuce leaves. Top with cashews and scallions, as desired.