Preheat your oven to 375°F (190°C) and lightly grease a small individual baking dish.
Cook the whole wheat lasagna noodles in boiling water until al dente, then drain and set aside.
Heat olive oil in a skillet over medium heat and sauté the diced onion and minced garlic until translucent.
Add the ground beef to the skillet, seasoning with sea salt, black pepper, and dried oregano, and cook until browned.
Stir the fresh spinach into the beef mixture until just wilted, then remove from heat.
In a small bowl, stir the ricotta cheese to soften it slightly.
Spread a spoonful of marinara sauce on the bottom of the baking dish, then layer with one noodle, half the beef and spinach mixture, half the ricotta, and more sauce.
Repeat the layer with the second noodle and remaining ingredients, finishing with the shredded mozzarella cheese on top.
Bake for 20-25 minutes until the sauce is bubbling and the cheese is golden and melted.