01 - Cook the udon noodles according to package instructions until tender. 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 softened and translucent.
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 the garlic.
05 - In a mixing bowl, whisk together white miso paste, soy sauce, sesame oil, heavy cream, and water until completely smooth and incorporated.
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 until noodles are evenly coated and spinach is just wilted.
08 - Taste and adjust seasoning as needed. Serve immediately in bowls, garnished with sliced green onions, toasted sesame seeds, and chili flakes if desired.