Cook the udon noodles according to the package directions until just loosened. Drain well and set aside.
Heat a large skillet over medium-high heat and add the oil. Add the sliced shiitake mushrooms and cook until softened and lightly browned.
Add the butter, garlic, and scallion whites. Cook for about 30 seconds, stirring frequently, until the garlic is fragrant.
Add the udon noodles, followed by the soy sauce, dark soy sauce, and chili oil. Toss everything together until the noodles are evenly coated and the sauce becomes glossy.
Transfer the udon to a bowl and top with an onsen egg. Finish with toasted sesame seeds, scallion greens, and an extra spoonful of chili oil if you like.
Right before eating, break open the onsen egg and mix the runny yolk into the hot noodles until everything gets extra creamy and saucy.
Calories: 272kcal, Carbohydrates: 9g, Protein: 4g, Fat: 26g, Saturated Fat: 9g, Polyunsaturated Fat: 3g, Monounsaturated Fat: 12g, Trans Fat: 0.5g, Cholesterol: 30mg, Sodium: 1195mg, Potassium: 277mg, Fiber: 2g, Sugar: 2g, Vitamin A: 470IU, Vitamin C: 4mg, Calcium: 28mg, Iron: 1mg