Step 1

Preheat the oven to 350 degrees F.

Step 2

In a large skillet, cook the beef and onion over medium heat until no longer pink, about 6-8 minutes.

Step 3

Add the garlic and cook until fragrant, about 1 minute.

Step 4

Drain the fat from the skillet.

Step 5

Stir in the spaghetti sauce, diced tomatoes, mushrooms, and Italian seasoning.

Step 6

Bring the mixture to a boil.

Step 7

Reduce the heat to low. Simmer, uncovered, for 20 minutes.

Step 8

Cook the pasta shells according to the package directions.

Step 9

Drain the pasta.

Step 10

Add the pasta shells to the beef mixture.

Step 11

Gently stir in the plum tomatoes.

Step 12

Transfer the mixture to an ungreased 9x13-inch baking dish.

Step 13

Top with provolone and mozzarella.

Step 14

Bake until bubbly and heated through, about 25-30 minutes.

Step 15

Serve warm.