How to Make It
In a small bowl, mix the mustard and water; let stand for at least 10 minutes. In another small bowl, combine the soy sauce, salt, sugar and sesame oil.
Bring a small saucepan of water to a boil. Add the broccoli and carrot and cook until crisp-tender, about 5 minutes. Drain well.
Heat a wok or large skillet over high heat. Add 1/2 tablespoon of the oil, swirling to coat the pan. Stir the eggs and add them to the wok. Let the eggs cook without stirring until the bottom is set, about 20 seconds. Stir until very loose large curds form, 10 to 20 seconds longer. Scrape the eggs onto a plate.
Add 1 tablespoon of the oil to the wok and heat until shimmering. Add the mushrooms and cook over high heat, stirring occasionally, until tender and beginning to brown, 5 to 6 minutes. Transfer the mushrooms to a plate. Add the remaining 2 tablespoons of the oil to the wok and heat until shimmering. Add the snow peas and stir-fry until crisp-tender, about 4 minutes.
Add the broccoli, carrot, mushrooms, rice, scallions and water chestnuts to the wok and stir-fry until heated through, about 4 minutes. Stir the soy mixture and drizzle it over the rice mixture. Add the scrambled eggs and toss gently but thoroughly. Transfer the fried rice to a bowl and pass the mustard separately.
Fried rice is best made with cold, leftover rice because it's not as sticky as fresh rice. Try to plan ahead and make a batch of white, long grain rice the night before or earlier in the day and keep it in the refrigerator until you are ready to use it.