Remove stem end of pumpkins by slicing about 1 1/2" off top. Scoop out seeds; reserve seeds of 2 pumpkins. Place pumpkins and tops, cut side down, on greased large baking sheets. Bake at 375° for 35 to 45 minutes or just until tender. Cool 30 minutes. Gently scoop out most of pulp from all 8 pumpkins, leaving enough pulp in 6 shells so they are intact for serving. Set aside 2 1/4 cups pulp.
Place onion and apple in a large saucepan; add 1 1/4 cups chicken broth. Bring to a boil; cover, reduce heat, and simmer 30 to 40 minutes or until very tender. Process pumpkin pulp and onion mixture in a food processor until smooth. Return mixture to saucepan; add remaining 1 cup chicken broth, half-and-half, curry powder, and salt. Cook over medium heat just until soup comes to a simmer. (Do not boil.)
Place pumpkin shells on individual serving plates. Spoon soup into each shell. Dollop with sour cream, and sprinkle with Spicy Pumpkin Seeds. Add pumpkin tops, if desired.