Preheat oven to 400°F. Bring a large pot of water to a boil for the pasta.
Heat large skillet over medium heat, add butter and melt. Add sage leaves, cook to light golden and crisp then remove—the butter will be brown and nutty. Add onions, garlic, squash and stock, bring to a boil then cover and cook to tender, about 15 minutes. Purée with an immersion blender until very smooth.