Melt butter in a large dutch oven or soup pot over medium heat. Add onions and a good pinch of salt and saute until completely soft and translucent, about 12 minutes. Add ginger and saute 30 seconds, until fragrant.
Stir in pumpkin, broth, cayenne and pepper, plus another good pinch of salt. Bring to a boil. Mash and break up the pumpkin chunks with a wooden spoon. Simmer 10 minutes. Taste the soup and add more salt a pinch at a time until it tastes vibrant and well seasoned. (The total amount will depend on how salty your broth and roasted pumpkin were).
Stir in cooked rice and simmer until heated through. Ladle into bowls and serve with chili oil.