Preheat your oven to 375°F (190°C).
In a small non-stick skillet, heat the olive oil over medium heat and sauté the diced onion and minced garlic until they are soft and fragrant.
Add the ground beef to the skillet, breaking it apart with a wooden spoon, and cook until it is thoroughly browned; drain any excess fat from the pan.
Stir in the tomato puree, dried oregano, sea salt, and black pepper, then let the sauce simmer on low for about 5 minutes to meld the flavors.
In a small mixing bowl, roughly chop the baby spinach and stir it into the ricotta cheese until evenly distributed.
Select a small, individual-sized oven-safe baking dish and spread two tablespoons of the meat sauce across the bottom.
Place one lasagna noodle over the sauce, then spread half of the ricotta-spinach mixture over the noodle, followed by a layer of meat sauce.
Add the second lasagna noodle, the remaining ricotta mixture, and the rest of the meat sauce.
Evenly sprinkle the shredded mozzarella cheese over the final layer of sauce.
Cover the dish tightly with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 10 minutes, or until the cheese is melted and begins to turn a golden brown.
Allow the lasagna to rest for 5 minutes before serving to let the layers set.