Mix together marinade ingredients in a medium container that can be covered. (Tupperware bowl). Add beef slices and coat in marinade. Marinate at room temperature for 30 minutes or cover and refrigerate until ready to use if marinating longer than 30 minutes.
Cook the noodles according to package directions or to basically just loosen up. They should take just a couple of minutes. Take care not to overcook as you want the noodles to soak up sauce during final cooking. Noodles come packaged curled over you can break them at the fold for an easier length to work with.
Heat 1 ½ teaspoons vegetable oil in a large nonstick skillet over high heat until hot and shimmering. Add enough of the beef slices to the skillet to cook in a single layer, cook without stirring for 1 minute, then flip beef over and sear just until browned, about 1-2 minutes. Do not overcook, it will still cook some more later, or it won’t be as tender! You’re just looking to brown it a bit here. Transfer beef into a bowl or plate and cover. Repeat with remaining beef slices as necessary.
Once beef is removed to the now empty pan add 2 teaspoons sesame oil over medium high heat. Add the onions, carrots and mushrooms and sauté for 3 minutes while stirring often. Add the bell peppers just at the end and stir-fry until crisp tender. I like the peppers crisp therefore I barely cook them.
Reduce heat to medium low. Add the noodles, beef and remaining sauce and cook while stirring to mix well and until noodles are warmed and sauce has thickened slightly, 4-5 minutes. Add spinach toss and cook and stir one minute, or just until spinach begins to wilt.
Taste and add additional hot chili oil if desired for more heat.
Garnish on top of serving with green onions and a sprinkle of sesame seeds.
The easiest way to cut thin beef slices is to freeze the beef so it is very firm but not frozen. Check after about an hour in the freezer. Then cut slices to about 1/8” across the grainDon’t cook beef all at once. Cook your steak in 2 batches so the pan is not overcrowded. You want the beef to sear your beef not to steam.Have all of the ingredients ready the veggies need to be sliced before you start your stir fry. It goes quickly.You can use almost any favorite veggie of your choice as long as it is not watery.If you use Rice Wine make sure it is Rice wine and NOT rice vinegar. The best substitute for rice wine is white dry cooking wine, which is what I used.When cutting carrots: I just use my peeler to make long deep cuts and then cut the strips into thirds. Very easy.