Bring water and salt to a boil. Add millet and return to a boil. Cover, reduce heat to low and simmer until cooked through, about 20 minutes.
Heat oil over medium-high heat in a wok or large pan. Add onion and cook until beginning to soften, about 5 minutes. Add garlic and ginger and cook for 1 minute. Add broccoli, carrots and water chestnut and cook until crisp-tender, 7–10 minutes. Add cashews and cooked millet.
Combine soy sauce, rice vinegar, honey and cornstarch and pour over vegetables. Cook until dressing is absorbed, about 2 minutes. Remove from heat and serve.