Creamy Curried Sweet Potato Soup

1 1/2 tablespoons olive oil 
1 cup coarsely chopped onion
1 large clove garlic, coarsely chopped 
1 tablespoon coarsely grated fresh ginger 
1 teaspoon ground cumin 
1/2 teaspoon ground coriander 
1/4 teaspoon ground cardamom 
1/4 teaspoon turmeric  
1/8 teaspoon crushed red pepper flakes (optional) 
2 1/2 pounds sweet potatoes, peeled and sliced ¼ inch thick
6 cups chicken broth, or more as needed 
Salt and freshly ground black pepper 
6 to 8 teaspoons fresh goat cheese 

Heat the oil in a pot large enough to hold all the ingredients. Add the onions and sauté over medium heat until it begins to brown, about 10 minutes. Add the garlic and sauté, stirring for 30 seconds. Add the ginger, cumin, coriander, cardamom, turmeric, and red pepper flakes, if using, and stir well. Add the sweet potatoes and broth and bring to a boil. Reduce the heat and simmer until the sweet potatoes are soft, about 20 minutes. 

Purée the soup in batches in a blender or food processor. Season to taste with salt and pepper. (The soup can be made ahead of time and refrigerated. Reheat slowly to serve.) If the soup is too thick, add a little more stock. 

Ladle into mugs, top each serving with a teaspoon of cheese, and stir to melt a little.