In a pot over medium heat, combine curry paste and 1⁄4 can coconut milk. Mix until paste is dissolved. Cut red pepper into small sticks. Finely chop red chili pepper (if using) and add to pot. Remove skin from chicken thighs; cut meat into small pieces and add to pot. When chicken is seared, add remainder of coconut milk, water and red pepper sticks. Set heat to high until mixture starts to bubble, then reduce heat to low and cook, uncovered, for 30 min. or until mixture has thickened.
Fill another pot halfway with water and bring to a boil. Add rice noodles; con-tinue cooking until noodles are tender. Drain and add noodles to curry pot. Add fish sauce (if using) and finely chopped cilantro. Cook for another 5 min., then add green onion. Dish into four serving bowls. If you like spicy food, top servings with more chopped red chili or red pepper flakes.