Preheat your oven to 375°F (190°C).
Cook the whole wheat lasagna noodles in boiling water according to package directions until al dente, then drain and set aside.
In a small skillet, heat the olive oil over medium heat. Add the diced onion and minced garlic, sautéing until translucent and fragrant.
Add the ground beef to the skillet, seasoning with half of the salt and pepper. Brown the meat thoroughly, breaking it into small crumbles.
Stir in the tomato puree and dried oregano. Lower the heat and let the meat sauce simmer for 5-7 minutes to develop flavor.
In a small bowl, finely chop the fresh spinach and mix it with the ricotta cheese, fresh parsley, and the remaining salt and pepper.
In a small oven-safe loaf pan or personal baking dish, spread a thin layer of meat sauce on the bottom.
Layer one noodle (trimmed to fit if necessary), half of the ricotta-spinach mixture, and a portion of the meat sauce.
Repeat the layer with the second noodle, the remaining ricotta mixture, and the rest of the meat sauce.
Top with the shredded mozzarella cheese and bake for 20-25 minutes until the cheese is bubbly and golden brown.
Let the lasagna rest for 5 minutes before serving to allow the layers to set.