Heat the butter in a large soup pot over medium heat. Sauté the onion and garlic until soft.
Add the carrots and cook until somewhat tender, stirring occasionally.
Pour in the stock, carrot juice and sugar. Season the soup with the salt.
Bring to a boil, lower the heat, and simmer, covered, until the carrots are very soft. This should take about 30 minutes. Remove from the heat and allow to cool.
Working in batches, transfer the carrot soup to a blender and puree until smooth.
Return the carrot puree to the pot and reheat over medium heat.
Stir in the lemon juice and season with pepper.
Divide among warm soup bowls and serve immediately.