Cook the udon noodles according to package instructions.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add sliced chicken or tofu to the skillet, cooking until golden.
Once cooked, add the sliced onion and garlic, stirring to combine.
Incorporate the carrots and bell pepper, cooking for another few minutes.
Add the cooked udon noodles to the skillet and toss everything together.
Drizzle soy sauce and oyster sauce over the mixture, stirring to coat evenly.
Cook for an additional two minutes until heated through.
Serve hot, garnished with chopped green onions.
