Step 1

Preheat the oven to 350 degrees F.

Step 2

In a large skillet, cook the beef and the onions over medium heat until the meat is 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, the diced tomatoes and their juices, the mushrooms, and the Italian seasoning.

Step 6

Bring the beef mixture to a boil.

Step 7

Reduce the heat to low and simmer, uncovered, for 20 minutes.

Step 8

Cook the pasta shells to al dente, 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 beef mixture to an ungreased 9x13-inch baking dish.

Step 13

Top the beef mixture evenly with the provolone and the mozzarella.

Step 14

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

Step 15

Serve warm.