Melt butter in a medium stockpot over medium heat. Add onion, garlic, and 1 teaspoon salt. Cook, stirring occasionally, until onion is translucent, about 5 minutes.
Place rosemary and sage on a small square of cheesecloth, and tie with kitchen string. Stir in the herb sachet, squash, potatoes, stock, and water. Bring to a boil, then reduce heat to a simmer, and cook until vegetables are soft, about 15 minutes. Discard herb sachet. Let cool slightly.
Working in batches, puree soup in a blender until smooth. Adjust to desired consistency with stock or water. (Soup can be made up to 2 days ahead; warm over low heat.) Season with salt and pepper. Serve with crostini.