Cook dried egg noodles as per instruction. Drain in a colander and run under running tap water to stop further cooking.
Toss chicken strips (cut against grain for tender chicken).
Mix together soy sauce, oyster sauce, fish sauce, and brown sugar.
Add 2 tablespoons of the above sauce to chicken strips, toss and let it rest for 15 minutes, while you prep the veggies.
Trim the end of lemongrass stem. Smash it with a heavy object and mince it well. Alternatively, grate it in a microplanner.
Slice onion and peppers.
Heat oil in a wok. Saute minced garlic and lemongrass for 30 seconds. Increase the heat, add marinated chicken strips (spreading in a single layer while adding to the wok). Cook it on medium-high heat undisturbed for 3-4 minutes.
Flip chicken. keeping the heat medium-high, quickly add sliced onion and peppers. Toss everything for 2-3 minutes more until the onions and peppers have softened a bit.
Add cooked noodles, birds eye chillis(if using) to the wok. Add remaining sauce around the edges of the wok ( watch video for reference). Toss everything quickly ( scarping the bottom of the pan to prevent the noodles from sticking) .
Switch off the heat. Add chopped green onion and fresh Thai basil. Give everything a good toss. Serve.