Brush cut side of squash with olive oil; sprinkle with salt, and pepper. Place squash, cut side down, on a lined baking sheet. Bake 15 minutes, or until tender. Scoop out pulp, keeping shells intact; reserve pulp. Reduce heat to 350 degrees F.
In a large skillet, melt butter over medium heat. Add onion; cook 5 minutes or until transparent. To the skillet, add stuffing mix, spinach, sour cream, Cheddar and squash pulp. Mix together and add salt and pepper, to taste. Cook for 3 minutes. Spoon mixture evenly into squash shells. Place on baking sheet, and bake 15 to 20 minutes, or until heated through.