Whisk together all ingredients for the sauce, set aside.
In a large skillet or wok, heat 1 tablespoon of oil over medium high heat. When hot, add the beef and cook for 2-3 minutes per side, until cooked through. This may need to be done in two batches to avoid crowding the pan. Remove beef from pan and drain off any excess liquid, wipe or rinse pan if needed.
Return pan to heat and allow any excess moisture to evaporate. Add remaining tablespoon of oil to pan.
When hot, add onion and cook for 2-3 minutes, stirring as needed, until onion is starting to soften.
Add bell pepper, mushrooms and white parts of bok choy and cook for another 3-4 minutes.
Add carrots and green parts of bok choy, cook for another 2 minutes
When vegetables are crisp tender, add the beef and noodles to the pan. Pour in the sauce, stirring to mix and coat everything in the sauce.
Cook for 1-2 minutes, until sauce has thickened.
Remove from heat. Garnish with sesame seeds and green onion if desired and serve.
I like to under cook the ramen noodles by about a minute so that they don't get mushy when added to the stir fry. It helps to add a dash of oil to the noodles after they are drained so that they don't stick together.