Cut squash in half lengthwise; discard seeds and membrane. Place squash halves, cut sides down, on a baking sheet; bake at 375° for 45 minutes or until tender. Cool. Peel squash; mash pulp. Set aside 3 1/2 cups pulp, reserving remaining squash for another use.
Melt butter in a large Dutch oven over medium-high heat. Add chopped pear and onion; sauté 10 minutes or until lightly browned. Add squash pulp, water, and next 5 ingredients (water through pepper). Bring to a boil; partially cover, reduce heat, and simmer 40 minutes. Place one-third of squash mixture in a blender; process until smooth. Pour puréed mixture into a large bowl; repeat procedure with remaining squash mixture. Return squash mixture to pan; stir in half-and-half. Cook over low heat 3 minutes or until thoroughly heated. Ladle soup into bowls, and garnish with pear slices.