Potato-leek soup is a blank canvas for all manner of garnishes. Keep it simple for family—chopped thyme and black pepper—and for a larger gathering, double the soup and set out an array of options so guests can add what they like. With a base of water, not broth, the soup is simple—and vegetarian. It gets richness from butter and a whole head of garlic that’s simmered until soft and blended in. It can be made, cooled and refrigerated in an airtight container for up to three days. Rewarm in a saucepan over medium-low, thinning with water as needed.