Pre-heat oven to 375°F.
Grease a deep 9-inch springform pan with cooking spray.
Bring a large pot of salted water to a boil; add the spaghetti and cook for 3 to 4 minutes less than the package recommends, so that it is very al dente; drain and reserve.
In a large skillet, over medium-high heat, brown the beef, 5 to 8 minutes; season with salt and pepper.
Transfer the beef to a paper towel-lined plate to drain.
Wipe the skillet clean with a paper towel.
Heat the olive oil in the skillet, over medium heat, and sauté the onions until translucent, 4 to 5 minutes; set aside.
In a large bowl, whisk together the marinara, ricotta, parsley, eggs, 1/2 cup of the Parmesan, 1 teaspoon salt and 1/4 teaspoon pepper.
Add the cooked spaghetti, beef and onions and toss to evenly coat.
Transfer to the prepared pan, top with the mozzarella and sprinkle with the remaining 2 tablespoons Parmesan.
Spray enough aluminum foil to cover the prepared dish with cooking spray; cover the dish with the aluminun foil, sprayed side down.
Place in the pre-heated oven and bake, covered with the foil, for the first 25 minutes; remove the foil; contiune cooking for another 25 minutes.
It is important to let the pie rest, at room temperature, for about 15 minutes before removing the springform pan sides, cutting into wedges and serving.
While the pie is resting, heat enough marinara sauce to liberally pour over each individual slice when serving; the additional sauce enhances the baked pie which has a tendency to become a little dry during the baking process.
