Start by bringing a large pot of salted water to a boil. Add the medium shells pasta and cook according to the package instructions until al dente. Once done, drain the pasta using a colander and set it aside.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté until it becomes translucent, about 3-4 minutes. Stir in the minced garlic and cook for an additional 30 seconds until fragrant.
Add the lean ground beef to the skillet with the onions and garlic. Cook, breaking it apart with a wooden spoon, until browned and cooked through. Drain any excess fat if necessary.
Sprinkle the flour over the cooked beef and stir well to combine. Add the Italian seasoning, salt, and pepper. Gradually pour in the beef broth and the can of tomato sauce, stirring continuously until the mixture is smooth.
Reduce the heat to low and add the cubed cream cheese. Stir until the cream cheese is fully melted and incorporated into the sauce.
Add the drained pasta shells to the skillet and mix everything together gently until the pasta is well coated in the sauce.
Sprinkle the shredded cheddar cheese over the top of the pasta mixture. Cover the skillet with a lid and let it sit for a few minutes until the cheese is melted and bubbly.
Once the cheese is melted, give it a final stir, and your 20 Minute Cheesy Beef and Shells is ready to serve! Dish it up hot, and enjoy this creamy, cheesy delight!