Step 1: Halve the leeks lengthwise and rinse away any grit. Thinly slice the leeks crosswise. In a small square of cheesecloth or hot tea sachet, tie together bay leaves, thyme, sage, parsley, and celery leaves if using.
Step 2: Melt the butter or heat the oil in the bottom of a large pot over medium-high heat. Stir in the leeks and cook, stirring frequently, until leeks are soft and dark golden brown, 10-15 minutes. Stir in the garlic for the last 3 minutes of cooking.
Step 3: Stir in stock or water, the sachet of herbs, the salt and pepper. Bring to a boil over high heat, then reduce down to medium-low; simmer for 5 minutes.
Step 4: Add the potatoes and simmer soup until potatoes are very tender and falling apart, about 45 minutes. Taste and adjust seasoning. If desired, drizzle small amount of cream into each bowl when serving, and top with parsley for garnish.