Pre-heat the oven to 375°F.
Bring a large pot of water to a boil over high heat. Once the water comes to a boil, add a generous handful of salt. Cook the pasta just short of al dente. Drain and return to the pot.
Pre-heat a medium size skillet over medium heat. Add the EVOO, onion, garlic and red pepper flakes. Cook, stirring frequently, for about 5 minutes. Add the beef stock and both cans of crushed tomatoes and bring to a bubble. Chop or crumble up the leftover meatloaf and add to the sauce. Simmer for 3-4 minutes. Season with salt and pepper, to taste.
Add half of the tomato meatloaf sauce to the cooked pasta. Add the ricotta and half of the Parmigiano-Reggiano cheese and toss to coat. Add half of the cubed mozzarella cheese, give everything a stir, then transfer it to an ovensafe baking dish.
Top with the remaining tomato meatloaf sauce, cubed mozzarella and the remaining Parmigiano-Reggiano cheese. Transfer to the oven and bake until the top is nice and brown, about 10-15 minutes.