Coat frying pan with sesame oil and fry the pork over medium-high heat. Add shiitake mushrooms and bamboo shoots, and continue frying. Pour in water and Chicken Flavor Udon seasoning and bring to a boil.
When it begins to boil, add in spinach and swirl in whisked egg. Add in vinegar and chili oil.
Remove udon noodles out of its packaging, and lay noodles into same pan from Step 2. Cook for 1-2 minutes.
Transfer contents to a bowl and ready to eat! *If you like, you can add a slurry of water and corn starch to Step 2 to thicken the soup. *If you like spicy, add 1 tsp of chili oil.