In a large pan, heat 4 tablespoons butter until it bubbles.
Then add the chopped leeks. Season with ½ teaspoon kosher salt and ½ teaspoon pepper. Stir occasionally for about 10 minutes until the leeks get really soft and the edges begin to brown.
Add garlic and stir for another couple of minutes. Taste before the next step.
Add 5 cups liquid (I use 50% chicken stock and 50% water) and the herbs. Taste and see if you like the balance so far. Some stocks are saltier than others.
Crank up the heat to bring it to a quick boil, then reduce the heat and simmer for 5 minutes.
Add 1 pound of thinly sliced Yukon gold potatoes and let it simmer with the lid on for about 40 minutes. Stir occasionally.
The soup is done when the potatoes are very tender and starting to fall apart. We might still have to add some more salt and pepper. Potatoes soak up some salt. Taste again. Season with salt until you achieve the desired yumminess. If you like, add a pinch of cayenne pepper, which really makes it pop.
Serve in a bowl, add a splash of heavy cream (if desired), and top with some fresh flat-leaf Italian parsley.
If you prefer a creamy consistency, use an immersion blender and blend everything together before serving.