In a small bowl, combine the water and cornstarch. Whisk in the lemon juice, honey, ginger and miso. Set aside.
Heat the oil over medium heat in a wok or heavy skillet and add the onion. Cook 2 min., then add the broccoli and tempeh.
Continue to cook 5 min. Stir in the mushrooms, snow peas, water chestnuts, and mirin and continue to cook 4 min.
Stir the cornstarch mixture to recombine, then add to the skillet and increase the heat.
Cook 2 more minutes, stirring frequently. Stir in the soy sauce, cook an additional minute, and serve hot.
Serve with steamed basmati rice.