In a large skillet over medium-high heat, cook the ground beef and chopped onion until the beef is browned and onions are translucent. Drain excess grease and stir in the minced garlic during the last minute of cooking.
Transfer the beef mixture to a 6-quart slow cooker.
Add the marinara sauce, beef broth, dry Ranch seasoning packet, and cubed cream cheese to the slow cooker. Stir well to combine.
Cover and cook on LOW for 4-5 hours or on HIGH for 2-3 hours.
30 minutes before serving, stir in the uncooked pasta shells. Ensure the pasta is submerged in the liquid as much as possible.
Cover and continue cooking on HIGH for 20-30 minutes, or until the pasta is tender.
Sprinkle the shredded cheddar cheese over the top. Cover for another 5 minutes until the cheese is fully melted and bubbly.
Stir gently before serving and garnish with fresh parsley if desired.