Preheat oven to 400ºF. In a roasting tray, place the butternut squash coat with 2 teaspoons olive oil, salt, pepper, & brown sugar and roast for 50 - 60 minutes or until the flesh is fork-tender. Set aside to cool slightly so it is easier to handle. Once you can handle the squash, peel the skin off and mash into small bits.
In a deep bottomed pan heat the rest of the oil on a medium-low flame and add in the minced garlic & sauté until the raw smell is gone. Add in the chopped sage leaves, sauté until fragrant, about 10 seconds. Then add in the squash, mix well, and sauté for a minute to allow the flavors to meld together.
Add in the stock, start with 1 cup at a time as you don't want to thin out the soup too much. Season with salt & pepper, and allow to heat through. With an immersion blender blend the mixture to a smooth consistency. You want to make sure there are no lumps and the soup has a smooth velvety consistency.
Add in the remaining stock, if needed, place a lid on the pan and bring to a gentle simmer for about 3 - 4 minutes. The squash itself is cooked well and all you want to do at this point is heat it through.
In the meantime, in a small pan heat the butter on a medium flame until it starts to sing and froth. You want to brown the butter slowly and not burn it so you have to be careful and watch it constantly. Once you start to see the butter change color to a golden brown add in the hazelnuts and toast for about a minute or until toasty and fragrant. Turn off the heat and add in the whole sage leaves and take off the heat and set aside.
Turn down the heat on the soup and add in the heavy cream and stir through. Once completely incorporated turn off the heat completely. You don't want the mixture to boil after the cream has been added in.
Spoon the soup into your serving bowls and top with the brown butter hazelnuts.