Slice beef into very thin strips, 1/4 inch thick and 2 inches long. Stir together 2 tablespoons soy sauce, 2 tablespoons peach nectar, grated ginger and cornstarch in a bowl until smooth. Toss the sliced beef with the soy sauce marinade to coat. Cover and refrigerate for 30 minutes.
Heat 1 teaspoon oil in a wok or large skillet over medium-high heat. Add beef with marinade; stir-fry until no longer pink, 2 to 4 minutes. Remove to a bowl, scraping out juices with rubber spatula.
Add the remaining oil, the chopped ginger, garlic, green onion and pepper flakes to wok or skillet; stir-fry 1 to 2 minutes. Add peach slices, water chestnuts and snow peas; stir-fry for another 2 minutes. Return beef to pan. Stir in remaining peach nectar, soy sauce and the vinegar. Serve immediately with hot cooked rice.