Preheat your oven to 400°F (200°C). Spread the chopped heirloom squash and garlic cloves on a baking sheet, then drizzle with olive oil and toss to coat evenly. Roast until the squash is tender and caramelized around the edges, about 25-30 minutes, filling the kitchen with a sweet, nutty aroma.
While the squash roasts, heat a tablespoon of olive oil in a large pot over medium heat. Add the chopped sage and sauté until fragrant and slightly crispy, about 1-2 minutes, causing a fragrant aroma to fill the air.
Once the squash and garlic are done roasting, carefully transfer the squash to the pot with the sage. Add the roasted garlic cloves, squeezing them out of their skins, and cook for a few minutes, stirring gently to combine the flavors.
Pour in the vegetable broth and bring the mixture to a gentle simmer. Cook uncovered for about 10 minutes to let the flavors meld and the squash soften further.
Use an immersion blender to blend the soup directly in the pot until smooth and velvety. Alternatively, carefully transfer the soup in batches to a blender, then return it to the pot.
Stir in the heavy cream if using, and season with salt and pepper to taste. Warm the soup gently over low heat, tasting and adjusting seasoning as needed.
Once heated through and well-seasoned, ladle the soup into bowls, garnishing with a few extra sage leaves or a drizzle of cream if desired. Serve hot, enjoying the rich, earthy flavors of the season.