In a large bowl, combine steak with salt, pepper, sesame oil and cornstarch. Allow to marinate for 30 minutes.
In a medium bowl, whisk together all the ingredients for the sauce and set aside.
Heat 1 tablespoon of cooking oil in a large skillet / saute pan on medium high heat until pan starts to get smoky. Add beef and allow to sear and cook until brown on all sides, about 1-2 minutes. Transfer onto a plate.
Return pan to heat. Add pineapples and cook until slightly browned and softened, around 5 minutes, Toss in carrots and beef then give the sauce a good stir then pour into pan, coating the ingredients on all sides.
*If cooking the zucchini noodles, add into pan and use tongs to toss and coat with sauce. Cook for 1-2 minutes until zucchini is just tender but still firm. Be careful not to overcook.
Serve immediately and garnish with green onions and sesame seeds if desired.
*do not overcook for longer than 2 minutes or else noodles will get too soft and mushy. You can also serve the beef over raw zucchini noodles if desired.