Place 1/2 chopped onion, ginger, garlic, and 1/4 cup water into a blender and puree until smooth. Soak rice noodles in a bowl of lukewarm water for about 30 minutes.
Season chicken thigh meat lightly with salt and black pepper in a bowl and set aside. In a separate small bowl, whisk 1 cup of chicken broth into the chickpea flour. Reserve remaining 2 cups chicken broth for a later step.
Heat vegetable oil in a skillet over medium-high heat. Stir in chicken pieces and cook, stirring frequently, until lightly browned. Remove the chicken pieces with a slotted spoon and set aside. Reduce heat to medium and add 1 1/2 sliced onions to the pan. Cook and stir until onion has softened and turned translucent, about 5 minutes. Reduce heat to medium-low and continue cooking and stirring until the onion is very tender and dark brown, 15 to 20 minutes more. Stir in the turmeric, paprika, and cayenne pepper.
In a medium stockpot, combine cooked chicken and any juices that have accumulated, browned onions, onion and ginger puree, and remaining 2 cups of chicken broth. Carefully add chickpea flour and broth mixture, stirring quickly so that the flour doesn't clump.
Bring the pot to a boil; reduce the heat to low and simmer, uncovered, for 15 minutes.
Stir in the coconut milk and cook just until heated through. Divide the soaked rice noodles among 5 large soup bowls. Ladle the soup over the rice noodles in the bowls and garnish each bowl with lemon wedges, green onions, hard-cooked eggs, cilantro, fish sauce, fried noodles, and curry powder.