Preheat oven to 350 °F.
Cut the neck off of squash and set aside. Cut base in half through root and scoop out seeds.
Place 1-2 sage sprigs in each half, drizzle lightly in olive oil, and put on baking sheet lined with parchment paper. Roast until very tender, 45-60 minutes. Remove from oven to cool.
When squash is cool enough to handle, discard sage, and scoop out flesh. Set aside.
Peel neck of squash and cut into 1/2” pieces.
Cut the onion, shallot, and carrot into a 1/2” dice.
Smash garlic cloves and roughly chop.
Cut leek, keeping only very light-green and white parts. Cut in half vertically, then into half-moon slices about 1/2” wide. Soak leek pieces in a bowl of water for 5 minutes, then lift out and place in colander to drain.
Wrap parsley, thyme, bay leaves, and peppercorns in a bundle of cheesecloth and tie with a string.
Add 2-3 tablespoons of olive oil to Dutch oven or soup pot over medium heat. Add the leeks, shallot, carrot, garlic, and sweat for 5-8 minutes until translucent.
Add diced squash and cook 4-5 minutes. Add honey/maple syrup and cook for 2-3 minutes more.
Add stock and herb bundle, turn heat to high and bring to a boil, then reduce heat until water is just simmering. Simmer for 15 minutes until squash pieces are tender.
Add roasted squash to pot and simmer for another 30 minutes.
Remove herb bundle. Add 3-4 tablespoons olive oil, and blend soup in batches (or with a stick blender) until smooth. Season with kosher salt and black pepper to taste.