In a large pot or dutch oven, heat oil over medium-high heat. Bruise (rub togethe4 of the sage leaves to release essential oils. Add them to the pot and cook for 30 seconds per side. This will infuse the oil with a mild sage flavor Remove them from the pot and discard. Reduce heat to medium.
Add diced onion and cook for about 2-3 minutes, stirring often. The object is to sweat the onions not brown them, so turn down the heat if needed. Add garlic, and cook for an additional 45 seconds.
Next, place stock, pumpkin, maple syrup, salt, black pepper, and pepper flakes to the pot. Let the mixture simmer for 30 minutes, stirring occasionally. Add half and half and a half and cook for an additional 5-7 minutes. Remove pot from heat and let stand for 5 minutes or so. Use an immersion blender to smooth out the soup. Taste for salt content and adjust as needed.
To serve, add 2-3 ladles full of soup into a soup plate or bowl, then top with a little olive oil, parmesan, and sage leave (either whole or finely diced)
Store leftover soup in an airtight container in the refrigerator for up to 5 days after preparation.