At its best, Italian cuisine creates a symphony of flavor from only a few simple ingredients. With rich ground beef, bright San Marzano tomatoes and a classic blend of Italian cheeses, these amazing stuffed shells perfectly embody this timeless culinary sensibility.
Preheat oven to 350 degrees F. Lightly oil a 15x10x1-inch baking sheet and a 3-quart oval or rectangular baking dish.
Bring a large pot of water, with 1 tablespoon salt added, to boiling. Add pasta shells and cook just until slightly tender, about 4 to 5 minutes. Drain pasta and spread in a single layer on baking sheet to prevent them from sticking together.
In a large skillet, heat 1 tablespoon of the olive oil over medium heat. Add beef and 1 clove of garlic. Season with salt and pepper. Cook and stir for 6 to 8 minutes, until no pink remains in beef. Transfer to a bowl; set aside.
For tomato sauce, in a second skillet over low heat combine remaining olive oil, remaining garlic, and undrained tomatoes. Season with salt and pepper. Bring sauce to a simmer over low heat; cook for 15 to 20 minutes until thickened somewhat, breaking up tomatoes with the back of spoon.
Stir 1 1/2 cups of tomato sauce into cooled ground beef, then add ricotta and 2/3 cup of the Parmigiano; stir until combined.
Spoon 1 1/2 cups tomato sauce into prepared baking dish. Fill each pasta shell with about 1 tablespoon meat and cheese mixture. Arrange shells in prepared baking dish. Spoon remaining tomato sauce over shells, then sprinkle remaining 2/3 cup cheese.
Bake about 30 minutes, until filling is heated through and top is golden brown.