Heat the oven to 425°F with the oven rack in the middle. Line a baking sheet with foil. Place the squash pieces cut-side up on the baking sheet. Melt 1 tablespoon of the butter and brush all of it over the tops and insides of the squash halves Roast in oven until knife tender, 50 minutes to 1 hour. Set aside to cool.
Peel, core and dice the apple. Melt remaining butter in Dutch oven over medium heat. Add the apple, onion, and sage, season with salt and pepper, and cook, stirring occasionally, until softened, about 10 minutes. Remove the pan from the heat and set aside.
Using a large spoon, scoop the flesh of squash into saucepan with apple mixture. Discard the squash skins. Add broth, water, and salt and pepper. Stir to combine thoroughly
Bring to a boil over medium-high heat. Reduce the heat to medium low and simmer, stirring occasionally, using a spoon to break up any large pieces of squash. Cook over medium-low for 20 minutes. Remove the pan from the heat and stir in the cream.
Using an immersion blender, work the blender around in the pot until the soup is smooth. A blender can also be used. Working in batches, puree until smooth. Taste and season with salt and pepper as needed. Garnish with Roasted Squash Seeds if desired.
** Add an additional apple if a sweeter soup is desired
**If soup becomes too thick, add small amounts of broth or cream until desired consistency is reached