Preheat your oven to 375°F (190°C).
Cook the whole grain lasagna noodles in boiling water according to package directions until al dente, then drain and set aside.
Heat the olive oil in a small skillet over medium heat and sauté the diced yellow onion and minced garlic until translucent 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, dried oregano, half of the sea salt, and half of the black pepper, then let the sauce simmer for 10 minutes.
In a separate small bowl, whisk the half egg and combine it with the ricotta cheese, chopped fresh spinach, and the remaining salt and pepper.
Spread a thin layer of the prepared meat sauce on the bottom of a small oven-safe baking dish.
Place a piece of the cooked lasagna noodle on top of the sauce, trimming it to fit the dish if necessary.
Spread half of the ricotta and spinach mixture over the noodle, followed by another layer of meat sauce.
Repeat the layers of noodle, ricotta mixture, and meat sauce until all ingredients are used, finishing with a layer of meat sauce on top.
Sprinkle the shredded mozzarella and grated parmesan cheese evenly over the top layer.
Cover the dish with foil and bake for 20 minutes, then remove the foil and bake for another 10 minutes until the cheese is bubbly and golden.