Wave goodbye to plain old store-bought boxed hamburger mixes and say hello to this family-pleasing casserole that combines pasta with everyone's favorite cheeseburger flavors. Second helpings will be requested!
Preheat the oven to 350°F.
Cook the spaghetti according to package directions, then drain and toss with 2 teaspoons olive oil. Let cool for about 10 minutes.
Brown the ground beef in a large skillet over medium-high heat, stirring occasionally, for about 5 minutes. Stir in the onion and cook, stirring occasionally, for about 5 minutes more. Stir in the tomato paste and cook for 1 minute more.
Remove the pan from the heat and pour the beef and onions into a medium bowl, scraping all the bits from the pan into the bowl.
Stir 2 tablespoons ketchup along with the relish and mustard into the beef and set aside.
In a large bowl, stir the remaining 2 tablespoons ketchup with the eggs, 1/2 cup of the cheddar, all of the mozzarella and salt. Add the pasta and gently toss to coat.
If using a deep 9- to 10-inch pie pan instead of the skillet in which you cooked the ground beef, lightly grease the pan with the remaining teaspoon olive oil.
Pour half the pasta mixture into the greased pan or back into the skillet. Top the pasta with the ground beef mixture, spreading it evenly. Top with the remaining pasta. Sprinkle the remaining 1/2 cup cheddar cheese over the pie.
Bake for about 30 minutes, until the pie is sizzling around the edges and the top of the pie is golden brown.
Let the pie rest for 10 minutes before slicing and serving.