Begin by cooking the rice noodles according to the package instructions. Once cooked, drain and set aside in a colander.
While the noodles are cooking, prepare the flank steak. Pat it dry with paper towels and season both sides with salt and black pepper.
Heat a large skillet over medium-high heat and add 2 tablespoons of butter. Once melted and bubbling, add the flank steak to the skillet.
Sear the steak for about 3-4 minutes on each side for medium-rare, or until desired doneness. Remove the steak from the skillet and let it rest for 5 minutes before slicing it thinly.
In the same skillet, reduce the heat to medium and add the remaining 2 tablespoons of butter and minced garlic. Sauté for about 1-2 minutes, or until fragrant but not browned.
Add the cooked noodles to the skillet along with soy sauce and oyster sauce (if using). Toss to combine and heat through for about 2-3 minutes.
Add the sliced steak to the noodle mixture and toss to coat everything in the garlic butter sauce. Adjust seasoning with salt and pepper if needed.
Serve the garlic butter steak lightning noodles hot, garnished with chopped green onions and fresh cilantro if desired. Serve alongside lime wedges for a zesty kick.