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Preheat oven to 375F.
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Bring a large pot of salted water to a boil. Cook the pasta shells according to the instructions on the package. Drain, rinse in cold water, and set aside.
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Heat olive oil in a large skillet on medium high heat. Add the onions and cook until softened, about 5 minutes. Add the sausage to the pan, breaking up the sausage with a wooden spoon. Cook sausage until cooked through, and no pink remains, about 5 minutes. Add the garlic and cook until fragrant, about 30 seconds to a minute more. Remove pan from heat.
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Beat the egg lightly in a large bowl. Mix in the ricotta, Parmesan cheese, basil, bread crumbs, salt, pepper, and sausage mixture. Fill each cooked pasta shell with some of the ricotta mixture.
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Spread ½ cup of pasta sauce over the bottom of 2 9x13-inch baking dishes. Arrange the stuffed pasta shells in the dishes. Spread the remaining sauce over the top of the pasta shells and top with mozzarella.
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Cover the pans with foil and bake for 30 minutes, until hot and bubbling. Remove foil and bake until the cheese is starting to brown, about 10 minutes.