If someone were to ask you, "What is Kenji doing right now?" at any given moment in the fall or winter, I'd put good odds on, "Making butternut squash soup." Why? Well, my wife, Adri, loves soup above all other foods, and her second-favorite soup* is butternut squash. My squash-simmering pants get worn frequently.
If you want to make the absolute best squash soup, you're best off following Daniel's advice and roasting your squash before souping it. This concentrates its flavor and gives the soup a natural, intense sweetness. But let's be honest: Your spouse doesn't always deserve the very best. Sometimes "just good enough" is good enough, so when I don't feel like cranking up the oven, I turn to this technique, which delivers a squash soup that's made 100% on the stovetop in just about half an hour.
The method is based on my master technique for creating creamy vegetable soups and it's pretty straightforward: sweat some basic mirepoix (onions, carrots, and celery) in butter, add some stock and a chopped butternut squash along with a couple bay leaves and thyme sprigs, let it simmer until tender, discard the bay leaves and thyme stems, then puree it all with a splash of heavy cream. Because you don't get the sweetness-enhancing effects of slow-roasting, and because squash can vary in sweetness, I'll sometimes add a bit of brown sugar or maple syrup to the soup after blending.
One quick and easy trick I've found that improves the flavor of the soup with almost no extra effort is to allow the butter to brown in the pot before adding the mirepoix. The nuttiness it imparts gives the soup some really nice depth of flavor.
When I was cooking the batch I made for photographs, I happened to have some leftover spiced nuts from the holidays (I used Daniel's Smoky Candied Almond recipe with pecans in place of almonds), as well as a little sage oil from a couple weeks back when Dave Arnold came over to demonstrate his Spinzall centrifuge prototype, so I used them. They were a wonderful match. Remember that for the next time you have sage oil from your countertop centrifuge lying around. (Or just do what I usually do: Drizzle some extra-virgin olive oil or browned butter on top.)
The great thing about this basic technique is that it easily adapts to suit your taste. Through years of failed experiments, I've learned that Adri likes her squash soup simple and without adornment, so that the flavor of the squash dominates. But her idea of fun might not be yours. You like some warm spices with your squash? Add a pinch of cinnamon, nutmeg, and clove just before adding the liquid. Try a squeeze of harissa paste in there if you like spice. Or add a dash of curry powder with coconut milk in place of heavy cream. Or maybe sprinkle in some smoked paprika. Or throw in a head of garlic and sauté it with the other vegetables.
So long as you keep the basic process and ratio of ingredients the same, you can take this soup in any number of directions.