Boil the shells: Preheat the oven to 400°F (204°C). Bring a large pot of generously salted water to a boil. Add the jumbo pasta shells and cook until just al dente (1–2 minutes less than package directions; usually about 8 minutes). Drain the shells well and set aside to cool.
20 jumbo pasta shells
Cook the sausage: While the shells are boiling, heat a large skillet or Dutch oven over medium-high heat. Add the Italian sausage and cook until mostly cooked through, breaking it apart with a wooden spoon.
1 pound Italian sausage
Make the sauce: Add the diced onion to the sausage and sauté 3–4 minutes until softened. Stir in the minced garlic and cook 30–60 seconds until fragrant. Pour in the marinara sauce and water, and reduce heat to low. Let the sauce simmer gently while you prepare the filling, stirring occasionally.
1 small yellow onion, 5–6 cloves garlic, 1 24-ounce jar marinara sauce, 1/4 cup water
Mix the filling: In a medium bowl, combine ricotta, egg, basil pesto, 1 cup of the shredded mozzarella, grated Parmesan, and kosher salt. Mix until smooth and evenly combined.
1 15-ounce container whole milk ricotta cheese , 1 large egg, 1/3 cup high-quality basil pesto, 2 cups shredded mozzarella cheese, 1 cup grated Parmesan cheese, 3/4 teaspoon kosher salt
Assemble the shells: Lightly grease a 9 x 13-inch (or 3.5-quart) oven-safe baking dish. Fill each cooled shell with about 2 heaping tablespoons of the ricotta mixture. Spoon the sausage marinara into the dish. Nestle them in the sauce with the slit side up.
Bake and broil: Cover the baking dish tightly with foil. Bake for 25–30 minutes, until the filling is heated through. Remove the foil and sprinkle the remaining 1 cup of shredded mozzarella on top. Broil 1–2 minutes, watching closely, until the cheese is golden and bubbly. Let the dish rest a few minutes before serving.