Preheat your oven to 375°F (190°C).
In a skillet over medium heat, brown the ground beef until fully cooked, then drain any excess liquid.
Pour the marinara sauce into the skillet with the beef and simmer on low for 5 minutes.
In a small mixing bowl, stir together the ricotta cheese, whisked egg, dried oregano, garlic powder, sea salt, and black pepper.
Boil the lasagna noodles in salted water according to package instructions until al dente, then drain.
Spread a thin layer of the meat sauce on the bottom of a small oven-safe baking dish.
Place a portion of a noodle sheet over the sauce, then layer with half of the ricotta mixture and half of the baby spinach.
Repeat the layers with the remaining noodles, ricotta, and spinach, finishing with the rest of the meat sauce on top.
Sprinkle the shredded mozzarella cheese evenly over the top layer.
Bake for 20 to 25 minutes until the cheese is melted and the sauce is bubbling around the edges.