Step 1

Grind saffron threads with a mortar and pestle to get 1/4 teaspoon ground saffron. Place the ground saffron in a small glass bowl. Add 2 tablespoons very hot (but not boiling) water. Stir, cover and set aside to steep.

Step 2

Heat oil in a large pot over medium heat. Add onion; cook, stirring, until soft and translucent, 6 to 8 minutes. Add garlic; cook, stirring, for another 2 minutes. Add squash, sweet potato, cumin, salt, cinnamon, coriander and pepper. Stir to combine; cook, stirring occasionally, until fragrant, about 5 minutes.

Step 3

Add 3 1/2 cups water to the pot; increase heat to high and bring to a boil. Reduce heat to medium-low and add the reserved saffron water. If there is saffron clinging to the side of the bowl, drizzle in a little more water, swish it around and add it to the pot (this is precious stuff!). Stir to combine; cover and simmer until the squash and sweet potato are soft and cooked through, about 25 minutes. Taste the broth for seasoning and adjust as desired.

Step 4

Transfer the soup to a blender and add maple syrup and orange blossom water, if using. (Start with 1 teaspoon maple syrup and just a drop or two of orange blossom water--and don't measure over the soup.) Puree until smooth (use caution when blending hot liquids). Taste and add more syrup and orange blossom water, if desired, but keep in mind the maple syrup and orange blossom water should not overwhelm the soup. Ladle into bowls and sprinkle with sumac, if desired.