Preheat your oven to 375°F (190°C).
Cook the whole grain lasagna noodles in boiling water according to package directions until al dente, then drain and set aside.
In a skillet over medium heat, add the olive oil and sauté the diced yellow onion and minced garlic until translucent and fragrant.
Add the ground beef to the skillet, seasoning with sea salt, black pepper, and dried oregano; cook until the beef is fully browned.
Stir the fresh baby spinach into the beef mixture until just wilted, then remove the pan from the heat.
In a small bowl, stir together the ricotta cheese with two tablespoons of the marinara sauce to create a creamy base.
In a small oven-safe baking dish, spread a thin layer of marinara sauce on the bottom.
Layer one lasagna noodle, half of the beef and spinach mixture, and half of the ricotta mixture.
Top with the second noodle, the remaining beef, the remaining ricotta, and the rest of the marinara sauce.
Sprinkle the shredded mozzarella cheese evenly over the top.
Bake for 20 minutes, or until the cheese is melted and bubbling with golden edges.
Let the lasagna rest for 5 minutes before serving to allow the layers to set.