Preheat your oven to 375°F (190°C).
Cook the lasagna noodles in a pot of boiling salted water until al dente, then drain and set aside on parchment paper to prevent sticking.
In a skillet over medium heat, sauté the diced onion and minced garlic in olive oil until soft and fragrant.
Add the ground beef to the skillet, seasoning with sea salt, black pepper, and dried oregano, and cook until fully browned.
Stir the fresh spinach into the beef mixture until wilted, then pour in the marinara sauce and simmer for 5 minutes.
In a small bowl, stir the ricotta cheese until it reaches a smooth, spreadable consistency.
In a small individual-sized baking dish, spread a thin layer of the meat sauce on the bottom.
Place one noodle over the sauce, followed by half of the ricotta cheese and another layer of meat sauce.
Repeat with the second noodle and remaining ricotta and sauce, then sprinkle the shredded mozzarella evenly over the top.
Bake for 20-25 minutes until the cheese is melted and the edges are bubbling.