Melt butter in large pot over medium-high heat and add onions and sauté until onions are softened, about 5-7 minutes.
Add butternut squash and sauté until squash softens, about 5 minutes.
Add garlic; stir 1 minute.
Add wine and sage leaves and stir until wine evaporates, about 5 minutes.
Add 4 cups of stock; bring to boil. Reduce heat, cover, and simmer until squash is very soft, about 25 minutes. Cool slightly.
Add ¼ cup of the Gruyère and using a handheld or standard blender, purée the soup (in batches, if necessary).
Season to taste with salt and pepper.
Return to a gentle simmer, stirring and adding more broth as necessary to achieve a thin soup with the consistency of heavy cream.
Position a rack about 6 inches from the broiler and heat the broiler on high.
Lay bread slices on a baking sheet. Toast under the broiler, 1 to 2 minutes per side. Sprinkle the croutons with the remaining 1 cup cheese and season with pepper. Broil until cheese melts and is bubbly and golden, about 2 minutes.
Ladle the soup into bowls and serve with croutons.
Recipe by Valerie's Keepers at https://www.valerieskeepers.com/butternut-squash-soup-sage-gruyere-toast/