2 tablespoons butter, for the baking dish
3 cups Marinara sauce
1 pound dried pasta shells, preferably large, about 1-1/2 inches
10 ounces cherry or grape tomatoes, cut in half
10 ounces fresh mozzarella, cut into 1-inch pieces
¾ cup Parmigiano-Reggiano, freshly grated
1½ cups mozzarella, shredded
¼ cup fresh basil leaves, shredded
Preheat the oven to 400 degrees. Butter the bottom and sides of the baking dish and spread half the marinara sauce in the bottom.
Cook the shells until slightly underdone, a minute or two less than package directions. While the pasta is cooking, heat the remainder of the sauce in the skillet.
Drain the cooked pasta in a colander, shake off excess water, and dump it into the pan. Toss pasta with sauce until coated, over medium-low heat, then toss with the sliced tomatoes for a minute or so to heat and soften them, but don't break them up.
Off the heat, toss the shells with half the grated cheese and turn everything into the baking dish. Scatter the fresh mozzarella balls or pieces and the shredded basil all over and fold them into the pasta.
Spread the pasta level in the dish, sprinkle the remaining shredded cheese all over and then the remaining grated cheese. Bake uncovered for 30 minutes or until the top is crusty and nicely colored.