Preheat your oven to 375°F (190°C).
In a skillet over medium heat, add the olive oil and minced garlic, sautéing for 1 minute until fragrant.
Add the ground beef to the skillet, breaking it apart with a spatula, and cook until browned.
Season the beef with sea salt, black pepper, and dried oregano, then stir in the fresh spinach until it is fully wilted.
In a small bowl, stir the ricotta cheese until smooth (add a teaspoon of water if needed to make it spreadable).
Spread a thin layer of marinara sauce on the bottom of a small oven-safe baking dish.
Layer half of a lasagna noodle (broken to fit), followed by half of the beef and spinach mixture, a dollop of ricotta, and more marinara.
Repeat the layers until all ingredients are used, finishing with a final layer of marinara and the shredded mozzarella cheese.
Cover the dish with foil and bake for 20 minutes; remove the foil and bake for an additional 5-10 minutes until the cheese is golden and bubbly.
Let the lasagna rest for 5 minutes before serving to allow the layers to set.