Beef Stir-Fry with Soba Noodles

In a large pot filled with water, cook soba noodles according to package instructions. Drain noodles (but reserve water) and transfer to a bowl. Drizzle with vegetable oil and toss together; set aside. Set up bowl of cold water with ice cubes. Cook green beans in soba noodle water for 2 minutes. Drain and rinse under cold water; transfer to ice bath. Meanwhile in a small bowl stir together soy sauce, brown sugar, rice vinegar, and mustard powder. In a separate small bowl mix together 2 tablespoons cornstarch and 1/4 cup water. Set aside. Rinse and pat dry beef. In a mixing bowl season with salt and black pepper; toss together with 2 tablespoons cornstarch until well coated. In a large sauté pan over medium-high add 2 tablespoons vegetable oil. When pan is hot but not smoking add beef and cook for 2 minutes on each side. Cook in two batches, then transfer to a plate. Add 1 teaspoon vegetable oil to sauté pan over medium-low heat. Cook garlic, onions, and ginger for 2 minutes. Pour in sauce and stir in drained green beans and beef; increase to medium-high heat. Stir cornstarch slurry and pour into pan. Mix until everything is well coated and toss together with soba noodles. Transfer to bowls and serve with scallion garnish.