Cook noodles as per package instructions. Reserve a cup of noodles cooking water.
Heat oil on a wok. Stir in garlic and white parts of scallion. Cook for a minute.
Add gochugaru and stir it. Reduce flame and 1/2 cup add noodles cooking water, gochujang, soy sauce, rice vinegar, and sugar. Stir and let the sauce cook for 2 minutes on low heat.
Add drained noodles, sliced green onion, toasted sesame seeds. Toss to coat noodles with the sauce.
Video
Notes
Gochugaru is Korean chilli flakes. Adjust the taste as per your spice tolerance. You can also skip it entirely.
Noodles cooking water helps thicken the sauce a bit.
I haven't added any extra salt, not even while cooking noodles. The sauces are already quite salty.
If adding any veggies like carrots, peppers, or bokchoy, add them in the order of their cooking time. Hard veggies will go in first followed by the rest.