One Pan Beef Stir Fry with Noodles

Name: One Pan Beef Stir Fry with Noodles
Ingredients: 1.5 pounds steak ( sliced 1/4 inch thick); 1.5 teaspoon cornstarch; 1 teaspoon salt; ½ teaspoon pepper; ⅛ cup peanut oil; 1 red onion ( cut into thin wedges); 1 red pepper ( sliced into 1/4 inch strips); 1 mead broccoli (cut into florets); 3 medium carrots ( sliced diagonal 1/4 inch thick); 8 ounces mushroom ( sliced 1/4 inch thick); 4 cloves garlic (minced); 1 Tablespoon fresh ginger (grated); 4 scallions; 2 bundles udon noodles (optional); 1/2 cup soy sauce; 1/4 cup brown sugar; 3 Tablespoons rice vinegar; 1/2 Tablespoon fish sauce; 1/2 Tablespoon sesame oil; 1/2 teaspoon red pepper (crushed )