Preheat oven to 160 ° C. Peel the onion, celery, ginger and 1 carrot, cut into 3-4 parts, crush the garlic cloves.
Rub the meat with salt and pepper. Preheat a large saucepan or pot, add 1 tbsp. l. butter and fry the meat until golden brown on all sides, transfer to a plate. Then add onions, carrots, garlic and celery and sauté, 5 minutes. Pour in mirin, broth, soy sauce, pepper paste and chili. Place back the pork to the pot and bring to a boil. Cover the pot with a lid and place in the oven for 4 hours. Turn the meat 2 times during cooking. Add hot water if necessary to maintain the volume of liquid.
Remove the cooked meat from the pan, remove the fat. Use two forks to cut into thin slices. Strain the broth from the pan through a sieve, remove the vegetables. Return broth and meat to saucepan and place over low heat.
Place the eggs in a medium size saucepan and add water until they are covered. Bring to a boil, then cook for 8-10 minutes. Transfer the cooked eggs to a bowl of cold water and peel.
Boil the noodles in boiling water, 5-6 minutes then drain them using a strainer and rinse water and set aside.
Chop the leeks thinly and sauté in oil, 5 minutes. Add spinach and cook for 1 more minute, season with salt and pepper. Cut the remaining carrots into thin long pieces.
Divide the noodles into deep bowls. Pour in hot pork broth, add leeks, spinach and carrots. Cut eggs in half and add to bowls. Sprinkle the ramen with green onions, sesame seeds, and chili flakes.