In a large pot, melt the cooking fat over medium heat, swirling to coat the bottom of the pot. When the fat is hot, add the onion and cook, stirring, until translucent, 2 to 3 minutes.
Add the squash, garlic and ginger and stir until the garlic becomes aromatic, about 1 minute.
Add the chicken broth and bring to a boil over high heat. Boil until the butternut squash is soft, about 10 minutes. Remove the pot from the heat.
In one or two batches, transfer the soup to a food processor or blender and blend on high speed until smooth in texture. Return the pureed soup to the pot.
Heat the soup over medium-high heat until it thickens enough to coat the back of a wooden spoon, 7 to 10 minutes.
Season with salt and pepper.