Preheat your oven to 375°F (190°C).
Bring a pot of salted water to a boil and cook the lasagna noodles according to package directions until al dente, then drain and set aside.
In a non-stick skillet over medium-high heat, brown the ground beef with the diced onion and minced garlic until the meat is fully cooked and onions are translucent.
Stir in the marinara sauce, dried oregano, sea salt, and black pepper into the beef mixture, then let it simmer for 5 minutes.
In a small bowl, finely chop the fresh baby spinach and stir it into the ricotta cheese until well combined.
In a small individual-sized baking dish, spread a thin layer of the meat sauce on the bottom.
Place one lasagna noodle over the sauce (trim to fit if necessary), then spread half of the ricotta-spinach mixture over the noodle.
Add another layer of meat sauce, followed by the second noodle, the remaining ricotta mixture, and the rest of the meat sauce.
Top the lasagna evenly with shredded mozzarella and grated parmesan cheese.
Cover the dish with foil and bake for 20 minutes, then remove the foil and bake for an additional 10 minutes until the cheese is golden and bubbly.
Allow the lasagna to rest for 5 minutes before serving to let the layers set.