Slice beef sirloin against the grain into thin strips for tenderness. Chop bell peppers, carrots, and snap peas.
Cook lo mein noodles in boiling salted water until al dente; drain and toss with a splash of oil to prevent sticking.
In a large wok over medium-high heat, sauté beef strips in oil for about 3-4 minutes until browned. Remove and set aside.
Stir-fry minced garlic and chopped vegetables in the same pan for about 2 minutes until slightly tender.
Return the beef to the pan along with noodles; add soy sauce, oyster sauce, and cornstarch mixture. Toss everything together for an additional 2 minutes.
Drizzle sesame oil on top before serving.