Melt butter in a saucepan over medium heat, and cook and stir the onion and garlic until the onion is translucent, about 3 minutes. Whisk in the chicken broth, and bring to a boil; whisk in the pumpkin puree, chipotle peppers and their sauce, milk, sage, salt, coriander, cinnamon, and nutmeg. Allow sauce to come to a simmer, then lower heat to hold at a simmer until serving time.