In your serving bowl, combine peanut butter, gochujang, soy sauce and rice vinegar.
Cook udon according to package directions, and at the end of the cooking time, reserve some of the cooking water if using frozen noodles. (I don't do this if using the shel-stable udon because it's coated in this weird film that seeps into the cooking water, so if that's what you're using then just boil water separately and use that)
Add hot water to sauce ingredients and whisk until smooth. Then add the udon and sesame oil, stir to coat the noodles and top with green onion and toasted sesame seeds.
