All meals deserve a finishing touch of something, and soup is no different. Flakey salt, a squeeze of lemon juice, or a smattering of fresh herbs are all welcome first-tier finishers, but the most luxurious of them all is a touch of fat, specifically butter or olive oil. It’s particularly special when used added to a bowl of soup.
I’m sure other fancy oils like hazelnut or sesame would work as well, but the ubiquitous olive works with almost any kind of soup, be it a hot bowl of creamy mushroom soup, or a chilled gazpacho. Butter is a bit more limited — it doesn’t do to well in cold soups — but very powerful when used correctly.
But before we talk soup specifics, it’s helpful to understand the benefit of finishing with a little fat. Fat carries flavour, but the flavour of the fat itself — like the tang in cultured butter, or the pepperiness of a good olive oil — can get lost and muddled during cooking. Adding a small amount to your soup right before serving lets that flavour shine, while also providing a fuller, more luxurious feeling in one’s mouth. (And who doesn’t want a mouthful of luxury?)
Olive oil is a pretty universal finisher, but I think it does particularly well with veggie-heavy gazpacho, or a “cream of” anything. It may seem like overkill to add oil to an already creamy soup, but high-quality olive oil feels silky, not creamy, and the two textures work in tandem to create a soup that feels ultra-rich but not too heavy or greasy.
Butter is a little different, and its application is a little more instinctual. Nothing really tastes bad with butter, though it would probably feel weird in a cold soup. Think about all the things restaurants slather on butter to heighten their flavour, then go from there. It can complement a beef and barley soup in the same way it finishes a steak, heighten the decadence factor in a bowl of chicken and rice (or stars), and — as A.A. Newton has mentioned previously — butter and miso go together “like peanut butter and jam.”
Baked potato soup, corn chowder, lentil soup, and ramen all become more meal-like and delicious with a little pat of butter floated on top. Adding it to a seafood bisque is probably overkill, but I’ve never been known for my restraint.