My Grandmother's Sautéed Beef with Onions and Noodles

Ingredients

• 1 pound flank steak, thinly sliced against the grain
• 2 tablespoons soy sauce
• 1 tablespoon rice vinegar
• 1 teaspoon cornstarch
• 2 tablespoons vegetable oil, divided
• 1 large onion, thinly sliced
• 3 cloves garlic, minced
• 2 cups bok choy, chopped
• 1 tablespoon oyster sauce (optional)
• 1 teaspoon sesame oil
• 8 ounces egg noodles
• Salt, to taste
• Fresh cilantro or green onions for garnish (optional)