Tie the thyme, bay leaf, and rosemary together with string or inside cheesecloth to create a bouquet garni. Set aside. Remove the outer layer of the leek. Cut off the tough upper leaves and the white part at the bottom. Under cool water, rinse any dirt off remaining section; slice into half-inch rings. Place leeks, carrots, potato, and bouquet garni into a pressure cooker or large pot with tight-fitting lid (such as a Dutch oven). Season with salt and pepper. Add enough water to just cover vegetables. Place on stove over medium heat; cover and simmer for approximately 15 to 20 minutes, or until vegetables are tender. Remove pot from stove and discard bouquet garni. Stir in butter. Working in batches, carefully pour vegetables and cooking liquid into a blender and puree until smooth. Divide soup among four bowls and garnish with rosemary sprig before serving.
