01 - Combine sliced beef with 1 tbsp soy sauce, cornstarch, and sesame oil in a bowl. Set aside to marinate while preparing other ingredients.
02 - Whisk together soy sauce, oyster sauce, chili garlic sauce, rice vinegar, brown sugar, and water in a small bowl until sugar dissolves. Set aside.
03 - Cook udon noodles according to package instructions. Drain and rinse under cold water, then set aside.
04 - Heat 1 tbsp vegetable oil in a large wok or skillet over high heat. Add beef in a single layer and stir-fry for 2 to 3 minutes until browned. Remove from pan and set aside.
05 - Add remaining 1 tbsp vegetable oil to the pan. Stir-fry garlic, ginger, and white parts of spring onions for 30 seconds until fragrant.
06 - Add bell pepper, carrot, and sugar snap peas. Stir-fry for 2 to 3 minutes until vegetables are tender but still crisp.
07 - Return beef to the pan. Add cooked udon noodles and pour in the prepared sauce. Toss for 2 to 3 minutes until noodles are heated through and evenly coated.
08 - Finish with green onion tops, toasted sesame seeds, and chopped cilantro as desired. Serve immediately.