Bring a large pot of salted water to a boil, add the egg noodles, and cook according to package directions until just tender. Drain well and set aside.

While the noodles cook, pat the beef sirloin dry with paper towels, then season it evenly with the salt and black pepper.

Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the seasoned beef in a single layer and cook for 3–4 minutes, stirring occasionally, until browned on the outside and just cooked through. Transfer the beef to a plate and set aside, leaving any juices in the pan.

Reduce the heat to medium and add the unsalted butter to the same skillet. Once melted, add the broccoli florets, carrot slices, red bell pepper strips, and sliced mushrooms. Cook, stirring often, for 5–6 minutes, until the vegetables are just tender but still slightly crisp.

Stir the minced garlic into the vegetables and cook for 30 seconds to 1 minute, until fragrant, being careful not to let it burn.

In a small bowl, whisk together the low-sodium soy sauce, oyster sauce, brown sugar, sesame oil, beef broth, and cornstarch until the cornstarch is fully dissolved and the mixture is smooth.

Pour the sauce mixture into the skillet with the vegetables. Stir well and bring to a gentle simmer, cooking for 2–3 minutes until the sauce thickens slightly and coats the vegetables.

Add the cooked egg noodles and the browned beef back into the skillet. Toss everything together over medium heat for 2–3 minutes, until the noodles are coated in the sauce and the beef is heated through.

Sprinkle the chopped green onion over the top, give the noodles one last gentle toss, and taste to adjust seasoning if needed before serving warm.