Prep the vegetables. Slice cabbage into chunks, carrot into thin strips, onion and chili into slices, and trim/slice mushrooms.
Fry the curry paste. Heat oil in a pot over medium-high, then fry the red curry paste until fragrant and the oil turns red.
Add the chicken. Stir in the chicken pieces until coated and no longer pink on the outside.
Simmer with coconut milk. Pour in coconut milk, water, fish sauce, and palm sugar. Simmer until the chicken is cooked through, and the broth is aromatic.
Add the vegetables. Stir in the cabbage, carrot, mushrooms, onion, and chili. Cook until just tender.
Prepare the noodles. Soak rice noodles in hot water until soft, then drain.
Combine and serve. Place noodles in bowls, ladle curry soup over the top, and serve with fresh herbs if desired.