Preheat your oven to 375°F (190°C).
Cook the whole wheat lasagna noodles in a pot of boiling water 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, browning it completely while breaking it into small crumbles with a wooden spoon.
Stir in the chopped fresh spinach and tomato puree, simmering for about 5 minutes until the spinach is fully wilted and the sauce has thickened slightly.
In a small mixing bowl, stir together the ricotta cheese, dried oregano, sea salt, and black pepper until well combined.
In a small individual-sized oven-safe baking dish, spread a thin layer of the meat sauce on the bottom.
Layer with a piece of lasagna noodle (break to fit), a layer of the ricotta mixture, and another layer of meat sauce.
Repeat the layers until all ingredients are used, finishing with a layer of meat sauce on top.
Sprinkle the shredded mozzarella cheese evenly over the top layer.
Bake for 20-25 minutes until the sauce is bubbling and the mozzarella cheese is melted and golden brown.