Preheat the oven to 425 degrees. Line a large, rimmed baking sheet with parchment paper.
Cut the butternut squash in half, lengthwise. Scoop out the seeds, but don’t throw them away.
Cut the onion into quarters, and cut just the tops off the heads of garlic.
Put the squash, cut side up, and the onions on the baking sheet. Pour just a touch of olive oil over the top of each head of garlic, sprinkle them a little sage, thyme, salt and pepper, and wrap them loosely in foil. Place them on the baking sheet, too.
Rub a coat of olive oil over the squash and onions, and then sprinkle them with sage, thyme, salt and pepper, too.
Roast the vegetables in the oven for 45 minutes or until the squash is fork tender.
Remove the baking sheet from the oven and set aside so the vegetables can cool enough for you to handle them.
In the meantime, rinse the seeds you removed from the squash to get rid of all the goopy strings. Dry them with a paper towel, and then put them in a small bowl, along with one tablespoon of olive oil and salt, to taste. Stir well to coat the seeds in oil and salt.
Heat a small skillet over medium heat. When it’s hot, add the seeds and let them brown, stirring regularly (be aware that they might try to jump out of the pan as they roast). Once they are lightly browned, remove the seeds from the heat, and set them aside.
If you have an immersion blender, heat a stockpot or Dutch oven over medium heat at this point. If not, you’ll want to have the pot ready but first add the ingredients to a blender and then transfer them to your pot after you puree them. (Remember to add a little stock to the blender with each batch so there’s liquid to help the blending process.)
Add the vegetable stock to the pot. Scoop out all the squash, discarding the skin, and put it into the pot, along with the roasted onions. Carefully squeeze as much of the roasted garlic out of each bulb as you can into the pot. Using your immersion blender, blend the soup until everything is smooth, and continue stirring regularly until the soup is hot. Stir in the cream, add any more seasonings, and serve with the roasted seeds sprinkled on top.