Prep the noodles. If using fresh or frozen udon, loosen in hot water for 1–2 minutes and drain. If using dried, boil per package until just tender, then rinse and drain.
Toss with a little oil to prevent sticking.
Mix the sauce. In a small bowl, combine gochujang, soy sauce, sugar, sesame oil, and kimchi juice (and vinegar if using). Adjust sweetness and heat to taste.
Heat the pan. Set a large skillet or wok over medium-high heat. Add oil.
Add aromatics. Stir-fry garlic and the white parts of the green onion for 20–30 seconds until fragrant.
Don’t brown the garlic.
Cook protein (if using). Add tofu, bacon, shrimp, or other protein. Stir-fry until cooked through or crisp. Remove and set aside if the pan gets crowded.
Stir-fry veggies. Add mushrooms, carrots, or other quick-cooking veggies.
Cook 2–3 minutes until just tender but still crisp.
Add kimchi. Toss in chopped kimchi and cook 1–2 minutes to concentrate flavor. Let it sizzle for a slight char.
Add noodles and sauce. Return protein (if set aside). Add udon and pour the sauce over.
Toss vigorously for 1–2 minutes until noodles are evenly coated and glossy.
Finish and taste. Add the green parts of the onions. Taste and adjust with extra soy for salt, kimchi juice for tang, or sugar for balance.
Serve. Top with sesame seeds, nori, and a fried egg if you like. Eat hot.