To make the pear slices, preheat your oven to 200° F. Line a rimmed baking sheet with parchment paper.
Cut the pear lengthwise into very thin slices, removing any seeds as you go. Lay the slices out in a single layer on your prepared baking sheet. Then transfer to your oven.
Bake until the pear slices are dry to the touch, 60 to 80 minutes. Remove and set aside.
For the soup
Warm 2 tablespoons of the olive oil in a large stockpot over medium-high heat. Add the cubed pumpkin, chopped pears, and onion. Cook, stirring occasionally, for 7 to 9 minutes, or until the ingredients are starting to soften. If the bottom of your pot starts to get dry, add the remaining tablespoon of olive oil.
Stir in the garlic, sage, salt, black pepper, and allspice. Cook an additional 2 minutes, or until fragrant. Then stir in the vegetable broth. Partially cover the pot with a lid so it's slightly ajar and bring the soup to a simmer. Cook until the cubed pumpkin is very soft, about 25 to 30 minutes. Turn off the heat and allow the soup to cool slightly.
Purée soup using an immersion blender, regular blender, or food processor with the blade attachment. If using a blender or food processor, work in batches as necessary and be careful when transferring the soup so as not to burn yourself.
Return the puréed soup to your pot and warm it over medium heat. Add the pumpkin butter and stir until fully incorporated. Taste and adjust for salt and pepper, if necessary.
To serve, ladle the warm soup into bowls. Garnish with the pear slices and fresh sage leaves. Enjoy immediately.
Leftover soup can be stored in an airtight container in your refrigerator for 3 to 4 days.