Make the marinade: In a small metal bowl, combine the rice vinegar, 1 Tablespoon ponzu, 1 Tablespoon sesame oil, garlic, ginger, and red pepper flakes. Cut the steak into very thin slices, and add to the marinade. Cover and set aside for 10 minutes.
Meanwhile, bring a large pot of water to a boil and prepare your noodles according to the package directions.
While the noodles cook, heat a large frying pan over high heat. When the pan is hot, add a little bit of sesame oil, then cook the peppers for about three minutes – they should be cooked, but still crisp. Remove the peppers from the pan and set aside. Return the pan to the burner. Add the steak. Cook for 30 seconds, stirring constantly. Immediately remove from the pan and set aside.
Reduce the heat to medium. Add the butter to the pan and heat until it is melted. Reduce heat to low and continue to cook until the butter turns light golden-brown. Slowly whisk in the remaining ponzu sauce. Reduce by half.
Drain the noodles, then toss with the brown butter and ponzu sauce and the peppers. Top with steak and garnish with green onions, sesame seeds, and orange zest.