Place the brown rice noodles in a bowl of very hot water and soak for 8-10 minutes until tender but firm, then drain.
In a small jar, whisk together the tamari, fish sauce, lime juice, and red pepper flakes to create the sauce.
Thinly slice the beef sirloin against the grain into bite-sized strips.
Heat avocado oil in a large skillet or wok over high heat; add the beef and sear for 2 minutes until browned and caramelized, then remove from the pan.
In the same pan, add minced garlic and sliced green onions, sautéing for 30 seconds until fragrant.
Push the aromatics to the side, crack the egg into the pan, and scramble quickly until just set.
Add the drained noodles, seared beef, and sauce to the pan, tossing everything together for 1-2 minutes until the noodles are well-coated.
Fold in the bean sprouts and cook for 30 more seconds to maintain their crunch.
Serve immediately topped with crushed peanuts and extra lime wedges if desired.