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 skillet over medium heat, add the olive oil and sauté the diced yellow onion and minced garlic until softened and fragrant.
Add the ground beef to the skillet, browning it thoroughly while breaking it into small crumbles with a spatula.
Stir in the tomato puree, tomato paste, dried oregano, sea salt, and black pepper, then let the sauce simmer for 5 minutes.
In a small mixing bowl, combine the part-skim ricotta cheese with the finely chopped fresh spinach.
In a small oven-safe baking dish, spread a thin layer of the meat sauce on the bottom.
Place a portion of the noodles over the sauce, followed by a layer of the ricotta-spinach mixture and more meat sauce.
Repeat the layers until all ingredients are used, finishing with a layer of sauce and a sprinkle of shredded mozzarella cheese.
Bake for 20-25 minutes until the cheese is melted and the edges are bubbling and golden brown.