Combine squash, 1/2 teaspoons salt, and water to cover in a Dutch oven. Bring to a boil, and cook 10 minutes or until tender. Cool.
Cut squash in half lengthwise, and remove seeds. Drizzle cut sides of squash evenly with 2 tablespoons melted butter; sprinkle evenly with 2 tablespoons cheese, 1/2 teaspoon salt, and pepper.
Pour remaining 2 tablespoons melted butter in a large skillet over medium-high heat, and add onion; sauté 4 minutes or until tender. Stir in cooked spinach, sour cream, red wine vinegar, and remaining 1/2 teaspoon salt. Spoon spinach mixture evenly into squash halves. Place squash in a 13 x 9 inch baking dish. Sprinkle with breadcrumbs and remaining 6 tablespoons cheese, and dot with cold butter.
Bake at 350 degrees for 15 minutes or until thoroughly heated.