Melt butter in a large saucepan over medium heat. Add mushrooms and onion and cook until tender, about 4-5 minutes, stirring often.

Stir in flour and curry powder and let cook, stirring constantly, for 1 minute. Gradually pour in chicken stock, continuing to stir until combined. Allow soup to simmer over medium heat, stirring constantly, until mixture is thickened, about 5-7 minutes longer.

Stir in pumpkin, honey, salt, nutmeg, and black pepper. Reduce heat to low and simmer for 10 minutes, stirring occasionally. Stir in evaporated milk and cook, stirring constantly, until heated through. Taste for seasoning and add more salt if needed.

Use an immersion blender to purée soup, if desired, before ladling into bowls. Serve soup topped with dollops of sour cream, a sprinkle of cilantro, additional fresh cracked black pepper, and a drizzle of chili oil, if desired.