Preheat oven to 350°. Cook pasta according to package directions for al dente. In a large cast-iron skillet, cook beef, onion and garlic over medium heat until beef is no longer pink, 6-8 minutes; drain and return to skillet. Stir in pasta sauce. Remove and set aside 1 cup meat sauce. Using the back of a spoon, make 10 shallow wells in meat sauce in the skillet; set aside. , In a large bowl, whisk eggs. Stir in cheeses and butter. Add cooked pasta to egg mixture and toss to coat. Twirl about 3/4 cup pasta mixture with a fork inside a measuring cup until a nest is formed. Transfer each nest to a well in the meat sauce. Widen the center opening using the handle of a wooden spoon. , Spoon a rounded tablespoonful of the reserved meat sauce into each nest. Top each with 2 rounded teaspoons ricotta. Cut 5 mozzarella cheese slices in half; arrange 1 piece over each nest (save remaining mozzarella for another use). If desired, season with salt, pepper, red pepper flakes and olive oil. , Cover and bake for 25 minutes. Uncover; bake until heated through, 10-15 minutes longer. Garnish with additional basil and grated Parmesan.