In a large pot or Dutch oven, melt the butter over medium heat. Add the onion and sauté for 5 minutes until tender and lightly golden, then add the garlic, flour, curry powder, coriander, and cayenne (if using) and cook, stirring often, for 1 more minute until you can smell the garlic and spices. Slowly add the broth, whisking all the while. Bring to a simmer over high heat, whisking frequently, then lower the heat and simmer for about 10 minutes until the soup has thickened a bit.
Whisk in the pumpkin and evaporated milk and continue whisking until the soup is creamy and fairly smooth. Add salt and pepper to taste. Serve hot, with croutons if desired.
Notes
To make vegan curried pumpkin soup, use vegetable oil instead of the butter, and replace the evaporated milk with a can of coconut milk.