Toast the cashew nuts in a dry skillet over medium heat, stirring frequently, until they turn golden and fragrant—about 3 to 4 minutes. Transfer to a plate to cool.
In a small bowl, whisk together lemon juice, honey, olive oil, salt, and pepper until the dressing is smooth and slightly emulsified.
Thinly slice the red onion and soak the slices in cold water for about 10 minutes to mellow their sharpness, then drain well.
In a large mixing bowl, toss the greens with the dressing until they are lightly coated, then add the soaked onion slices and cooled toasted cashews.
Gently fold the ingredients together, ensuring the greens are evenly coated without bruising, and check the seasoning, adding more lemon or salt if desired.
Serve the salad immediately for the best crunch, or refrigerate for up to 30 minutes if needed, but enjoy it fresh for optimal texture.