Preheat the oven to 375°F (190°C).
Cook the lasagna noodles according to package instructions.
In a skillet, cook the ground beef over medium heat until browned.
Add the minced garlic and cook for an additional minute.
Stir in the spinach and cook until wilted, about 2-3 minutes.
In a bowl, combine ricotta cheese, egg, Italian seasoning, and a pinch of salt and pepper.
Spread a layer of marinara sauce on the bottom of a baking dish.
Layer three lasagna noodles over the sauce.
Spread half of the ricotta mixture over the noodles.
Add half of the beef and spinach mixture on top.
Sprinkle with a third of the mozzarella and Parmesan cheeses.
Repeat layers, starting with marinara sauce, noodles, ricotta, beef mixture, and cheese.
Finish with a final layer of noodles topped with marinara sauce and remaining mozzarella and Parmesan cheeses.
Cover with foil and bake for 25 minutes.
Remove foil and bake for an additional 15-20 minutes, or until golden and bubbly.
Let the lasagna rest for 10-15 minutes before slicing.