Preheat your oven to 375°F and prepare a small individual-sized baking dish.
Cook the whole wheat lasagna noodles in boiling water until al dente, then drain and set aside.
In a medium skillet, heat the olive oil over medium heat and sauté the diced onion and garlic until translucent.
Add the ground beef to the skillet, seasoning with sea salt, black pepper, and oregano, and cook until fully browned.
Stir the marinara sauce and baby spinach into the beef mixture, simmering until the spinach has wilted.
Spread a thin layer of the meat sauce at the bottom of the baking dish, followed by a layer of noodle, a dollop of ricotta, and more sauce.
Repeat the layers until all ingredients are used, finishing with a sprinkle of mozzarella cheese on top.
Bake for 20 minutes, or until the sauce is bubbling and the cheese has turned a beautiful golden brown.