Preheat your oven to 375°F (190°C) and bring a small pot of salted water to a boil.
Cook the whole grain lasagna noodles until al dente, then drain and set aside on a piece of parchment paper to prevent sticking.
In a medium skillet, heat the olive oil over medium heat and sauté the diced onion and minced garlic until translucent and fragrant.
Add the ground beef to the skillet, breaking it up with a spoon, and cook until browned and fully cooked through.
Stir in the tomato puree, fresh spinach, dried oregano, sea salt, and black pepper; simmer for 5 minutes until the spinach has wilted and the sauce has thickened slightly.
In a small bowl, stir the ricotta cheese with a pinch of extra oregano and pepper until smooth.
In a small individual-sized baking dish, spread a spoonful of the meat sauce on the bottom, followed by half a noodle sheet (cut to fit).
Layer on half of the ricotta mixture, another layer of sauce, and another noodle sheet; repeat until all ingredients are used, finishing with a layer of meat sauce.
Sprinkle the grated parmesan cheese evenly over the top.
Bake for 15-20 minutes until the cheese is melted and the sauce is bubbling around the edges.