4 pounds whole butternut squash (about 2 medium), halved lengthwise and seeds removed
2 tablespoons unsalted butter (1/4 stick)
1 medium Granny Smith apple (about 8 ounces)
1/2 medium yellow onion
8 fresh sage leaves
2 1/2 cups low-sodium vegetable or chicken broth
2 1/2 cups water
1 1/2 teaspoons kosher salt, plus more as needed
1/4 teaspoon freshly ground black pepper, plus more as needed
1/3 cup heavy cream
1/2 cup toasted pumpkin seeds, for garnish (optional)