Whisk together the ingredients for the sauce. Toss the beef with some cornstarch separately. Then, brown the steak on both sides in vegetable oil, then remove it to a plate.
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Add the onion, celery, carrots, and mushrooms, then cook. Add the snow peas in the last minute of cooking, followed by the garlic and water chestnuts. Move the veggies to the plate with the cooked beef.
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Whisk the sauce again and add it to the skillet, then cook until thickened. Add the cooked veggies and beef, then toss to coat. Serve.
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