Preheat the oven to 350℉. Spray a 9x13-inch casserole dish with nonstick cooking spray. Set aside.
Bring a large pot of salted water to a boil over high heat. Once boiling, add the jumbo shells and cook to the al dente time according to package directions. When the shells are done cooking, drain them and rinse them under cold water; set the shells aside.
1 box (12 oz) jumbo shell pasta
In a large skillet pan, over medium-high heat, cook and crumble the ground beef and onion. Cook until the meat is browned and no longer pink, and the onions are softened.
2 pounds lean ground beef, 1 onion
Stir in the garlic and cook for an additional 30 seconds. Remove any extra grease from the skillet pan before proceeding.
4 cloves garlic
Add the Italian seasoning, onion powder, garlic powder, kosher salt, and black pepper to the beef mixture and stir everything together well.
1 tablespoon Italian seasoning, 1 teaspoon onion powder, 1 teaspoon garlic powder, 1 teaspoon kosher salt, ½ teaspoon black pepper
Stir in the crushed tomatoes and tomato sauce and continue cooking for about 5-8 minutes so the flavors can blend together. If needed, reduce the heat if it starts to splatter.
1 can (28 oz) crushed tomatoes, 1 can (15 oz) tomato sauce
Spread a thin layer of the sauce inside the prepared pan. Fill the shells with the meat mixture, and add about 1 tablespoon shredded mozzarella cheese to each shell. Place the filled shells inside the prepared baking dish, in a single layer, on top of the sauce.
1 block (16 oz) mozzarella cheese
Pour the remaining meat mixture and remaining sauce over top the filled shells inside the casserole pan. Spread the remaining cheese over top.
Bake for 20 minutes or until the cheese is melted and the casserole is bubbly.
Serve the shells warm from the oven. Enjoy!