In a heavy skillet over medium-high heat, toast the Indian coriander and Grains of Paradise, stirring occasionally, until fragrant and lightly browned, about 3-4 minutes. Transfer to a small bowl an allow to cool. Grind in a mortar & pestle or electric mill.
In a large Dutch oven, melt the butter over medium heat. Stir in the onion, salt and sugar and allow to cook about 5-7 minutes until translucent and aromatic. Add the garlic and 2 teaspoons of the spice mix and cook about 30 seconds more.
Add the sweet potatoes, vegetable broth, water and peanut butter and bring to a boil. Reduce the heat to low and simmer until the sweet potatoes are fork tender, about 25 minutes.
Turn off the heat and allow the soup to cool slightly. Working in batches so the blender is never more than half full, transfer the soup to a countertop blender and puree until smooth.
Return the soup to the pot and warm gently before serving. Garnish with a sprinkle of fried onions and a sprinkle of the remaining spice mix.
Notes:
Adapted from Cook's Illustrated