Add the olive oil to a large Dutch oven set over medium-low heat. Heat for 4-5 minutes until the oil is shimmering but not smoking. Add the onions and saute, stirring frequently to make sure they don't brown.

When the onions are soft and translucent, add the ginger and cook for an additional minute. Pour the chicken stock (or water) into the pot and add the canned sweet potato. Increase the temperature of the burner to high heat and bring everything to a boil, stirring frequently.

Reduce the heat again, stirring again to make sure that all of the sweet potato is broken up and well mixed into the stock (or water). Add the coconut milk and fish sauce and simmer for 30-40 minutes on low heat. Season to taste with at least 1 teaspoon salt and 1 teaspoon pepper.

Remove the soup from the stove and let it cool slightly. Pour the soup into a blender or food processor and pulse for 10-15 seconds in batches to get a really smooth consistency. Pour into bowls and garnish with kaffir lime leaves and lime wedges.