Preheat the oven to 425 °F (218 °C). Get out a large baking sheet and grease it a bit with olive oil.
Add the butternut squash, olive oil, sage leaves, salt and pepper to the baking sheet. Toss to combine.
Bake for 15 minutes and flip each piece over.
Sprinkle over the walnuts and garlic. Bake for another 10 minutes.
Serve warm. Refrigerate leftovers in an airtight container for up to 3 days.