Cook squash in salted water. Drain water and press squash with a fork or a potato ricer. Set aside.
Heat 3 tablespoons of oil and 1 tablespoon of butter in a medium size skillet and sauté leek until tender. Add ½ cup wine and reduce until liquid is evaporated. Combine with squash puree and set aside.
Bring stock to simmer in a large pan.
Heat rest of oil and sauté onion and garlic until soft but not brown. Add rice and stir well 1 minute. Add remaining wine and stir constantly until absorbed. Add stock, ½ cup at a time, and simmer until liquid is absorbed. Continue adding stock, stirring constantly and letting each addition be absorbed before adding the next, until rice is just cooked and mixture is creamy. Add hot squash and leek mixture, and stir well. Add cream, parmesan cheese and sage. At the end, before serving, add remaining butter. This process takes approximately 20 minutes. Season to taste.
Serve immediately. Sprinkle over with fresh sage.