Preheat broiler. Place squash, cut sides down, in a microwave-safe dish. Cover with plastic wrap. Microwave 15 minutes or until fork-tender. Use a fork to remove spaghetti-like strands from squash.
Meanwhile, cook bacon in a large skillet over medium heat 5 minutes or until crisp. Drain, reserving drippings in skillet. Crumble bacon.
Add onion and garlic to hot drippings; cook 2 to 3 minutes or until onion is tender. Stir in cream cheese, broth, and salt; cook until smooth.
Gradually add spinach; cook until thickened and spinach is wilted. Stir in squash strands. Spoon mixture into squash shells. Sprinkle with Cheddar and bacon. Broil 2 to 3 minutes or until cheese is golden.