First, heat the oven to 425°.
Line two baking sheets with foil.
Lay the squash on one sheet and use a paring knife to stab it in the bottom, making sure the knife gets all the way in to the cavity where the seeds are (until you don't feel resistance). This incision will let steam escape as it cooks.
Then roast the squash for 60-90 minutes, depending on its size, until a knife easily slides through the center.
When the squash has about 30 minutes left in the oven, cut the apples in half from top to bottom, then cut in half again so you have 4 wedges. Cut out the seeds and stem.
Peel, halve and roughly chop the onion.
Add the apples and onion to the 2nd foil-lined baking sheet and toss with the oil and ¼ teaspoon salt. Roast them in the lower rack of the oven for about 25-30 minutes, until they're soft and caramelized.
Once the squash is done, remove from the oven and cut it in half lengthwise and split it open to release the heat. Scoop out and discard the seeds.
Add the roasted apples and onion to a large soup pot. Scoop the butternut squash flesh out of the skin (if the squash is still too hot, hold it with a towel) and add to the pot.
Add the broth, 1¾ teaspoons salt, cardamom and pepper and use a stick blender to blend everything together until smooth.Or, if using a regular blender, add half the vegetables and half the broth, remove the inner insert from the blender's lid and hold a folded-up towel over the hole. Blend until smooth and pour into the pot. Repeat the process with the remaining vegetables and broth. Put the pot over medium-high heat and once the soup is warmed through, taste to adjust seasonings and serve.