Cook the lasagna noodles. Bring a large pot of salted water to a boil. Add the lasagna noodles (broken into roughly 2-inch pieces) and cook until al dente according to package directions. Drain and transfer them to a large mixing bowl.
Prep the meat sauce. Meanwhile, heat the olive oil in a large skillet over medium heat. Add the onion and cook for 5 to 7 minutes, stirring frequently, until very soft and lightly golden. Add the ground beef and cook for 6 minutes, or until browned. Add the garlic and cook for 20 seconds. Stir in the marinara sauce, Italian seasoning, salt, and black pepper. Bring to a simmer and cook for 10 minutes.
Prepare the spinach. While the sauce simmers, roughly chop the spinach. Place it in a dry skillet over medium heat and cook for 1–2 minutes until wilted. Transfer to a plate and let cool slightly. Once cool enough to handle, squeeze out as much liquid as possible and chop once more.
Make the ricotta mixture. In a mixing bowl, combine the spinach, ricotta, egg, nutmeg, salt, pepper, Pecorino Romano, ¼ cup Parmesan, and goat cheese. Mix until combined.
Prep for baking. Preheat the oven to 350ºF. Lightly grease a 13×18-inch sheet pan.
Assemble the lasagna. Pour the meat sauce into the bowl with the cooked pasta. Add 1 cup mozzarella and 1 cup Parmesan. Mix until everything is evenly coated. Spread the pasta mixture evenly in the sheet pan. Then, dollop spoonfuls of the ricotta mixture all over the top. Sprinkle with the remaining mozzarella and Parmesan.
Bake the lasagna. Bake for 30-40 minutes, or until bubbly and golden brown.
Serve. Remove the lasagna from the oven and let it cool for 10 minutes before serving.
Notes
Instead of fresh spinach, you can use thawed, thoroughly drained frozen spinach.
Do not overcook the noodles. The lasagna is baked additionally, so you do not want to end up with mushy noodles.
The sauce may look thinner, but that is okay. The lasagna noodles need extra liquid to cook up nicely.
Dollop, don't spread. Add dollops of ricotta to get creamy pockets of cheese. Do not smear it.