Prepare the Noodles: Boil a large pot of water. Cook fresh egg noodles for 2-4 minutes or dried egg noodles for 5-7 minutes. Drain and rinse under cold water; set aside.
Sauté Onions and Garlic: Chop onions and garlic. Heat oil in a pan over medium-low heat. Sauté onions for 5-6 minutes until caramelized. Add garlic and sauté for 2-4 minutes until golden.
Make the Soup: Stir in yellow curry paste and cook for 2 minutes. Add chicken stock cube, cardamom powder, turmeric powder, and coconut milk. Add chopped chicken and a drop of fish sauce; simmer for 20 minutes. Optionally, add Kefir lime leaves and lemongrass.
Fry the Noodles and serve: Heat oil in another pan and fry egg noodles until golden and crispy. Place khao soi in a bowl, sprinkle with parsley, add chili slices, and top with fried noodles. Enjoy!