01 - Cook the udon noodles according to package instructions until al dente. Drain thoroughly and set aside, keeping warm.
02 - Heat neutral oil in a large skillet over medium heat. Add sliced onion and cook for 2 minutes until translucent and softened.
03 - Add sliced shiitake mushrooms to the skillet. Sauté for 3 to 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 the garlic.
05 - In a mixing bowl, whisk together white miso paste, soy sauce, sesame oil, heavy cream, and water until completely smooth and well incorporated.
06 - Pour the sauce mixture into the skillet with the vegetables. Bring to a gentle simmer and cook for 2 to 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 wilted.
08 - Taste the dish and adjust seasoning if needed. Serve immediately in bowls, garnished with sliced green onions, toasted sesame seeds, and chili flakes if desired.