Heat the oil in a pot over medium heat. Add the leeks and garlic and cook until they are softened, about 4-5 minutes. Mix in flour until well combined. Slowly stir in the stock. Mix in the potatoes and thyme. Bring to a simmer and cook until leeks and potatoes are very tender, about 20 minutes. Puree in a food processor, and then return to the pot. Bring back to a simmer. Whisk in whipping cream; season with salt and pepper. Pour into soup bowls. Sprinkle cheese on top and serve.