Heat the olive oil in a large pot over a medium-high setting.
Sauté the onions for about 5 minutes, until fragrant and softened.
Add the ginger and garlic, and continue to sauté for another 2 to 3 minutes.
Sprinkle the curry powder and Cayenne pepper, and cook for another minute or two.
Add the chopped carrots to the pot, along with the lemon zest and juice, and chicken or vegetable stock.
Reduce the heat to a low setting, cover the pot, and simmer for about 20 minutes. The carrots should be very tender at this point.
Puree the soup in batches in a blender, or use an immersion blender.
Return to the pot to reheat before serving. If it’s too thick for your taste, you can add a little more vegetable or chicken stock at this point.
Top with a dollop of sour cream to serve.