Cook the Udon Noodles:
Boil the Udon noodles in a large pot of water according to the package instructions (about 3-4 minutes). Drain and set aside.

Stir-Fry the Vegetables:
Heat sesame oil in a large wok or skillet over medium-high heat. Add the minced garlic and ginger and sauté for about 30 seconds. Then, add the onion, bell pepper, carrot, and cabbage, and stir-fry for 5-7 minutes until they’re tender but still crisp.

Add Protein and Sauce:
Add your chosen protein (chicken, beef, tofu, or shrimp) to the pan and cook until fully cooked. Then, add the soy sauce, mirin, and oyster sauce (if using). Stir everything together.

Combine with Noodles:
Add the boiled Udon noodles to the pan and toss them with the vegetables and protein until everything is well combined and the noodles are coated with the sauce. If desired, sprinkle in chili flakes for extra heat.

Garnish and Serve:
Garnish with chopped green onions and serve hot.