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, diced onion, and minced garlic, sautéing until translucent.
Add the lean ground beef to the skillet, seasoning with sea salt, black pepper, and dried oregano. Cook until browned and crumbled.
Stir in the tomato puree to the beef mixture and let it simmer on low for 5 minutes to meld the flavors.
In a small bowl, finely chop the fresh spinach and mix it thoroughly with the part-skim ricotta cheese.
In a small individual-sized baking dish, spread a thin layer of the meat sauce on the bottom.
Place half of a noodle sheet 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 final layer of meat sauce.
Sprinkle the shredded mozzarella cheese evenly over the top.
Cover with foil and bake for 15 minutes, then remove the foil and bake for an additional 10 minutes until the cheese is bubbly and slightly golden.
Let the lasagna rest for 5 minutes before serving to allow the layers to set.