Preheat your oven to 425°F. Trim the top and bottom ends of the squash, and stand it upright on a cutting board. Cut the squash lengthwise down the middle. Scoop out and discard the seeds from each half.
Brush the cut sides of the squash with a thin coat of olive oil, then season with salt and pepper. Place the halves cut-side down on a baking tray, and roast for 45-55 minutes, until the squash is tender and caramelized.
Allow the squash to cool a bit, then scoop out the flesh and discard the skin. Set the squash aside for later.
In a large pot, heat 2 tbsp of butter and 1 tbsp of olive oil over medium heat. Add the leeks, along with a pinch of salt and pepper. Cook until the leeks are soft (10-12 minutes).
Add the garlic and chopped sage and cook for another 60 seconds, stirring frequently.
Add the butternut squash and chicken broth. Bring to a simmer and cook for 10-15 minutes. Remove from heat.
Blend the soup using an immersion blender, or regular blender, until smooth and creamy. Season to taste with salt and pepper. Depending on the type of blender you have, you may want to let the soup cool a bit before blending (see notes below).
Optionally garnish with fried sage leaves, creme fraiche, sour cream, roasted pumkin seeds, or whatever other toppings you might enjoy. (see notes below)