Preheat the oven to 375 degrees F (190 degrees 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.

Meanwhile, heat remaining 3 tablespoons oil in a large Dutch oven over medium-high heat. Add sirloin and sausage and cook, undisturbed, until bottom side is lightly browned, about 6 minutes. Stir, and continue to cook, stirring and breaking up meat into smaller pieces with a wooden spoon, until cooked through, about 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 about 2 cups of the ragu in the bottom of a 13- x 9-inch baking dish. Spoon 1/2 of the pasta over sauce. Dollop half of the ricotta mixture over pasta, and sprinkle with 1 cup mozzarella cheese. Spoon remaining pasta over mozzarella cheese. Spread remaining ragu over pasta mixture, and dollop with remaining ricotta mixture. Top with remaining 1/2 cup mozzarella cheese. Cover lasagna with a large sheet of aluminum foil that’s been lightly sprayed with cooking spray (to prevent the cheese from sticking to it).

Bake in preheated oven for 30 minutes. Remove from oven and remove foil. Increase oven temperature to broil with rack about 10 inches from heat source; preheat 5 minutes. Return baking dish to oven. Broil until cheese is browned in spots, 5 to 8 minutes.