01 - Cook udon noodles according to package directions until al dente. Drain thoroughly and set aside.
02 - Heat neutral oil in a large skillet over medium heat. Add sliced onions and cook for 2 minutes until translucent and softened.
03 - Add sliced shiitake mushrooms to the skillet. Sauté for 3–4 minutes until tender and lightly browned.
04 - Stir in minced garlic and grated ginger. Cook for 1 minute until fragrant, being careful not to burn.
05 - In a mixing bowl, whisk together white miso paste, soy sauce, sesame oil, heavy cream, and water until completely smooth and emulsified.
06 - Pour the sauce mixture into the skillet with vegetables. Bring to a gentle simmer and cook for 2–3 minutes, stirring frequently until slightly thickened.
07 - Add cooked udon noodles and baby spinach to the skillet. Toss everything together using tongs until noodles are thoroughly coated and spinach is just wilted.
08 - Taste and adjust seasoning if necessary. Serve immediately in bowls, garnished with sliced green onions, toasted sesame seeds, and chili flakes if desired.