Saute the 4 slices of bacon in a large pot until it is crispy. Using a slotted spoon remove the bacon to a paper towel to drain.
In the same pan, melt the 2 tablespoons of unsalted butter. Add the 4 large leeks - white and light green parts only and cook over low heat until soft, about 5 minutes.
Increase the heat to medium-high. Add the 1.5 lbs of Idaho russet potatoes, 1.5 lbs of Yukon gold potatoes, 1 bay leaf, 1 of 2 sprigs of fresh thyme, and 4 cups of chicken or vegetable stock. Bring to a boil. Reduce heat to low, cover and simmer for 20 - 30 minutes or until the potatoes have softened.
Remove the bay leaf and thyme. Remove ⅓ of the potato mixture to a bowl. Using a hand blender, puree the remaining soup until smooth. Return the potato mixture to the pot.
Season with salt and pepper to taste and serve hot with your desired toppings.
Notes
TipsWhen chopping the potatoes, cut them uniformly so that they cook in the same amount of time.If you don't have an immersion/stick blender you can also puree the soup in a food processor or blender. If you're using a blender, purée the soup in batches. Don't fill the blender more than a third full at a time, and remember to hold down the lid while the blender is going.If you don't have a blender, cook the soup as instructed and use a potato masher to mash the potatoes in the soup after they have softened.To make this leek and potato soup vegetarian, skip the bacon and increase the butter to 4 tablespoons. Replace the chicken stock with vegetable stock.If you prefer a thicker soup reduce the amount of chicken stock. If you prefer a thinner soup increase the amount of chicken stock.StorageIf you have leftovers, allow the soup to come to room temperature and then store it in an airtight container in the refrigerator for 3-4 days.