In a small bowl, whisk together soy sauce, oyster sauce, sesame oil, and brown sugar. Set aside.
Cook the lo mein noodles according to package instructions. Drain, rinse with cold water to stop cooking, and toss with a teaspoon of oil to prevent sticking.
In a large wok or skillet over medium-high heat, add 1 tablespoon of oil. Add the sliced beef in a single layer. Sear for 2-3 minutes until browned but not fully cooked through. Remove beef from the pan and set aside.
Add the remaining 1 tablespoon of oil to the pan. Add onions, carrots, broccoli, and green beans. Stir-fry for 3-4 minutes until vegetables are crisp-tender.
Add the minced garlic and grated ginger to the vegetables. Cook for 30 seconds until fragrant, being careful not to burn the garlic.
Return the beef (and any juices) to the pan along with the cooked noodles.
Pour the sauce mixture over the noodles and beef. Toss everything together for 1-2 minutes until the noodles are evenly coated and heated through.
Stir in the cornstarch slurry if a thicker sauce is desired and cook for another minute.
Remove from heat. Garnish with sesame seeds and serve immediately.