Make the stock and keep it warm over low heat.
Heat 1 tb. of the olive oil and butter in a soup pot; add the carrots, half tsp. salt and a pinch of pepper. Saute over medium heat until the carrots begin to glisten and release their juices, about 10 minutes. A 1 cup stock, stirring and scraping the bottom of the pot to loosen the sugars. A 3 more cups of the stock and the potatoes; bring the soup to a gentle boil, reduce heat and simmer, covered, for about 30 minutes, until the carrots are very tender. Add 2 more cups of stock and puree in a blender or food processor until smooth. Return to the pot and cook over low heat.
While the soup is cooking, cut leek in half lengthwise, wash and thinly slice. Heat 1 tb. olive oil in a saute pan and add the leeks, garlic, half tsp. salt, dried thyme and a pinch of pepper. Cover and cook over medium heat until the leeks are tender, 5 to 7 minutes. (The water on the washed leeks will help to wilt them.)
Add the leeks to the carrot puree and cook, uncovered, over low heat for another 30 minutes. Add cream if you're using it and season with salt and a few pinches of pepper to taste. Garnish each serving with Gruyere cheese and sprinkle with fresh thyme.
Make 9 to 10 cups.
TIP: If the flavor of the carrots is lacking, add a sweet potato to the soup to sweeten and bring out the flavors.
NOTE: Make this smooth, light soup in the spring with sweet carrots and tender, pale green leeks. It's the carrots that make this soup delicious, so use the most flavorful ones you can find.