Step 1: In a large skillet, heat 1 tablespoon of olive oil over medium-high heat. Add the lean ground beef and diced onion. Cook, breaking up the beef with a spoon, until the beef is thoroughly browned and the onion is softened, approximately 7-8 minutes. Drain any excess fat from the skillet.
Step 2: Stir in the minced garlic to the beef and onion mixture and cook for just one more minute until fragrant. Transfer the entire beef mixture to a 6-quart (or larger) slow cooker.
Step 3: To the slow cooker with the beef, add the beef broth, tomato sauce, diced tomatoes (undrained), Ranch seasoning mix, and black pepper. Stir everything together thoroughly, ensuring all ingredients are well combined.
Step 4: Cover your slow cooker with its lid and cook on the LOW setting for 4-6 hours, or on the HIGH setting for 2-3 hours.
Step 5: About 30-45 minutes before serving, stir the uncooked medium pasta shells directly into the slow cooker. Make sure the pasta is fully submerged in the liquid. Continue to cook on high until the pasta is al dente, tender with a slight bite.
Step 6: Once the pasta is cooked, stir in the heavy cream and the shredded cheddar cheese. Stir continuously until the cheese is completely melted and the sauce becomes rich, creamy, and lusciously thick.
Step 7: Ladle generous portions into bowls. If desired, garnish with fresh chopped parsley for a pop of color and freshness. Serve immediately and savor every creamy, cheesy bite!