Cook the udon noodles according to the package instructions. Once cooked, drain and set aside.
In a small bowl, mix together the gochujang, soy sauce, sesame oil, garlic, ginger, heavy cream (or coconut milk), sugar, and rice vinegar until a smooth sauce forms.
Heat the vegetable oil in a large skillet over medium-high heat. Add the onions and sauté until they are translucent and fragrant.
Add the mixed vegetables to the skillet and stir-fry for 3-4 minutes, until they are tender-crisp.
Pour the gochujang sauce into the skillet with the vegetables, bringing the mixture to a simmer. Allow the sauce to cook for 2-3 minutes, ensuring it thickens slightly.
Gently add the cooked udon noodles to the skillet, tossing them to evenly coat with the sauce and vegetables. If using a protein, add it to the skillet at this point and mix thoroughly.
Continue cooking for another 2-3 minutes until everything is heated through and well incorporated.
Serve the creamy gochujang udon noodles hot, garnished with sliced green onions and toasted sesame seeds.
