Follow these steps for a delicious, melty casserole:

1. Preheat and prepare dish
   Preheat your oven to 375 °F (190 °C). Grease a 9×13​‑inch baking dish with butter, oil, or nonstick spray.
2. Cook the pasta
   Bring a large pot of salted water to a boil. Add the pasta shells and cook according to the package directions until al dente. Drain and set aside.
3. Brown the beef mixture
   While the pasta cooks, heat a large skillet over medium​‑high heat. Add the ground beef, diced onion, and minced garlic. Season with salt and pepper. Brown and crumble the beef until no longer pink and the onion softens.
4. Combine with sauce and pasta
   To the skillet, add the tomato soup, beef broth, Italian seasoning, and half of the shredded cheese (reserve the rest). Stir to combine. Then gently fold in the cooked pasta shells. Continue cooking and stirring for 2–3 minutes over medium heat so everything melds.
5. Transfer to baking dish
   Pour the beef​‑pasta mixture into your prepared 9×13 dish, smoothing it out evenly. Sprinkle the remaining cheddar and mozzarella cheese over the top in an even layer.
6. Bake covered, then uncovered
   Cover the baking dish tightly with foil. Bake for 20 minutes at 375 °F. Then remove the foil and bake an additional 10–15 minutes, until the cheese is melted and bubbly and edges are slightly golden.
7. Rest and serve
   Allow the casserole to rest for 5–10 minutes before serving — this gives it time to firm up, making it easier to slice and serve neatly.