Trim the carrots and chop them into rough 1/2-inch chunks. Trim the root-end off the leeks and cut the white section into quarters. Slice across into small 1/2-inch pieces. (Save the greens for homemade stock.) Place the chopped leeks in a colander and rinse thoroughly—they are often sandy.
Place a large pot over medium heat. Add the butter, carrots, leeks and garlic and saute for 10 minutes, stirring regularly.
Then add the thyme, lemon zest, stock, wine, 1 teaspoon salt, and pepper to taste. Cover and bring to a boil. Stir and cover again. Simmer for 20-25 minutes, until the soup looks murky and the carrots are soft.
Ladle the soup into a high-powered blender. (You might have to do this in two batches.) Place the lid on the blender and open the top vent for steam. Lay a dish towel over the top of the blender and hold firmly as you turn the blender on. Pressure from blending hot liquids can blow the top off and burn you, so be careful—open vent, cover with towel, hold tight! Puree until smooth. Then add the sour cream to the blender and puree again.
Pour both batches back into the sauce pot and stir to blend. Serve warm.