Preheat the oven to 375°F (190°C). Set a rack in the center of the oven, about 10 inches from the heat source.

Bring a large pot of salted water to a boil. Add shells and cook, stirring occasionally, until almost tender, about 7 minutes. Drain pasta in a colander. Return to pot and toss with 1 tablespoon oil.

Heat remaining 3 tablespoons oil in a large Dutch oven over medium-high heat. Add sirloin and sausage and cook without stirring until the bottom side is lightly browned, about 6 minutes. Stir and continue breaking up the meat until cooked through, about another 4 minutes.

Reduce heat to medium. Add onion and garlic, and cook, stirring often, until softened, about 4 minutes.

Stir in marinara sauce, basil, and crushed red pepper. Remove ragu from heat.

Stir together ricotta, Parmigiano-Reggiano, cream, and salt in a bowl.

To assemble: Spread 2 cups of ragu into the bottom of a 13 x 9-inch baking dish. Spoon half of the pasta over the sauce, dollop with half of the ricotta mixture, and sprinkle with 1 cup mozzarella cheese. Add remaining pasta, ragu, ricotta, and top with remaining mozzarella cheese.

Cover lasagna with a large sheet of sprayed aluminum foil to prevent sticking. Bake in preheated oven for 30 minutes.