Lasagna is a dish that can be made ahead and can feed a crowd; it’s as easy to make for twelve as it is for four. I actually have lasagna parties where I make both the classic version and a fantastic vegetarian version, and my guests mix and match. If you make the lasagna ahead of time and serve it family style with a simple green salad as a side, you can feed a whole crowd while not spending more than five minutes in the kitchen during your party—literally, only five minutes. It can take longer just to give directions to the pizza-delivery guy.
Bring a Large Pot of salted water to a boil. Add the vegetable oil (the oil will help prevent the lasagna noodles from sticking together). Cook the lasagna until almost al dente, about 6 minutes (the center of the pasta should remain somewhat hard so that it won’t overcook and become soggy when baked). Drain, then rinse the noodles under cold water to stop the cooking and help remove any excess starch. Cover lightly with a damp towel to prevent the pasta from drying out. Set aside.
In a large sauté pan, heat the olive oil over a medium-high flame. Add the ground beef and ½ teaspoon each of salt and pepper, and sauté until the beef is brown, breaking up any large clumps, about 8 minutes. Remove from the heat and drain any excess fat. Let cool completely.
Position the rack in the center of the oven and preheat the oven to 375 degrees F.
In a medium bowl, mix the béchamel and marinara sauces to blend. Season the sauce with more salt and pepper to taste.
In another medium bowl, mix the ricotta, eggs, and ½ teaspoon each of salt and pepper to blend. Set aside. Spread the butter over a 13x9-inch baking dish. Spoon one third of the béchamel-marinara sauce over the bottom of the dish. Arrange 5 lasagna noodles atop the sauce, overlapping slightly and covering the bottom of the dish completely. Spread the ricotta mixture evenly over the noodles. Top with the spinach. Arrange 5 more lasagna noodles atop the spinach, then top with the ground beef. Spoon one third of the remaining béchamel-marinara sauce over, then sprinkle with ½ cup of mozzarella cheese. Top with the remaining 5 sheets of lasagna noodles. Spoon the remaining béchamel-marinara sauce over the lasagna noodles, then sprinkle with the remaining mozzarella cheese and the Parmesan cheese. (The lasagna can be made up to this point 1 day ahead. Cover tightly with plastic wrap and refrigerate. Uncover before baking.)
Line a large, heavy baking sheet with foil. Place the baking dish on the baking sheet. Bake until the lasagna is heated through and the top is bubbling, about 45 minutes.