Preheat the oven to 400°F. Toss the cubed butternut squash with 2 tablespoons olive oil, salt, and pepper on a baking sheet. Spread the cubes in a single layer and roast for 25 to 30 minutes until tender and lightly browned at the edges.

While the squash roasts, heat the remaining tablespoon of olive oil in a large pot over medium heat. Add the diced onion and cook for 5 minutes until softened.

Stir in the minced garlic and 6 sage leaves. Cook for 1 minute until fragrant.

Add the roasted squash and vegetable broth to the pot. Bring the mixture to a simmer and cook for 10 minutes.

Remove the pot from the heat. Use an immersion blender to puree the soup until smooth, or transfer it in batches to a countertop blender.

Taste and adjust salt or pepper if needed. Chop the remaining 2 sage leaves and stir them in just before serving.