Cook pasta shells according to package directions; drain and rinse with cold water.
Meanwhile, cook pancetta in oil in a large skillet over medium heat until crisp. Remove to paper towels, reserving drippings. Saute the spinach, garlic and pepper flakes in drippings until spinach is wilted.
Transfer spinach mixture to a small bowl. Add ricotta, Parmesan cheese, egg yolk and pepper; mix well.
Spread 1/2 cup Alfredo sauce into a greased 11-in. x 7-in. baking dish. Spoon ricotta mixture into pasta shells; place in baking dish. Pour remaining sauce over shells.
Cover and bake at 375° for 25 minutes. Sprinkle with mozzarella cheese. Bake 10-15 minutes longer or until cheese is melted. Top with pancetta. Yield: 6 servings.