Preheat your oven to 375°F (190°C).
Boil the whole-wheat lasagna noodles in a pot of water until al dente, then drain and set them aside on a piece of parchment paper to prevent sticking.
Heat the olive oil in a medium skillet over medium heat and sauté the diced yellow onion and minced garlic until they become translucent and aromatic.
Add the lean ground beef to the skillet and cook until fully browned, breaking it into small crumbles as it cooks.
Stir in the tomato puree, dried oregano, sea salt, and black pepper, then add the fresh baby spinach and stir until the leaves are just wilted.
In a small mixing bowl, stir together the part-skim ricotta cheese and half of the grated parmesan cheese.
Spread two tablespoons of the beef and tomato sauce across the bottom of a small, individual-sized oven-safe baking dish.
Place one lasagna noodle over the sauce, trimming to fit if necessary, then spread half of the ricotta mixture and half of the remaining beef sauce over it.
Repeat the layer with the second noodle, the rest of the ricotta mixture, and the remaining beef sauce.
Top the lasagna evenly with the shredded mozzarella and the remaining grated parmesan cheese.
Bake for 20 to 25 minutes until the cheese is melted, golden brown, and the sauce is bubbling around the edges.