Preheat oven to 425°F.
Peel the squash, cut in half and remove seeds and pith. Cut each half in half again. Place squash, quartered onion and garlic in single layer on roasting pan.
Drizzle the vegetables with olive oil; coating well. Sprinkle the ground cumin on the squash pieces and then place the squash cut side down in the roasting pan.
Sprinkle vegetables with salt and pepper. Scatter 3/4 of the sage leaves in the pan.
Roast for 30 minutes, turning vegetables occasionally so they do no burn. Continue roasting for approximately 30 minutes longer or until vegetables can be pierced easily with a fork. (The onion and garlic will take less time so you can remove them earlier from the oven.) Remove squash and set aside.
Transfer vegetables to a large soup pot. Add the onions; squeeze out the roasted garlic cloves from their peel. Next add the squash. Discard sage. Process soup mixture using a hand held immersion blender. Puree until smooth.
Stir in vegetable broth. Heat soup until very hot, but not boiling. Add the vinegar or lemon juice, salt and pepper to taste and the whipping cream if using.
Serve soup hot sprinkled with roasted pumpkin seeds if desired and a few small thinly sliced sage leaves for garnish.