Cook udon noodles according to package instructions. Drain and set aside.

In a skillet, heat vegetable oil over medium-high heat. Add onion, garlic, ginger, and red bell pepper and cook for 2-3 minutes, until the onion is soft and translucent.

Add shiitake mushrooms to the skillet and cook for another 5-7 minutes, until the mushrooms are tender.

In a small bowl, whisk together vegetable broth, soy sauce, and rice vinegar.

Add the broth mixture to the skillet and bring to a simmer. Add bok choy and scallions and cook for 2-3 minutes, until the bok choy is wilted.

Add the cooked udon noodles to the skillet and toss everything together until well combined.

Garnish each serving with sesame seeds and serve hot.