Preheat your oven to 375°F (190°C).
In a skillet over medium heat, sauté the diced yellow onion and minced garlic until fragrant, about 2 minutes.
Add the ground beef to the skillet, seasoning with sea salt and black pepper, and cook until browned; drain any excess fat.
Stir in the marinara sauce, dried oregano, and fresh spinach, simmering until the spinach is wilted and the sauce is well combined.
In a small bowl, stir together the ricotta cheese and parmesan cheese until smooth.
Slice the zucchini into very thin rounds to act as extra nutrient-dense layers.
In a small individual-sized baking dish, spread a spoonful of meat sauce on the bottom, then layer with half of the broken lasagna noodle, half of the zucchini slices, half of the meat sauce, and half of the ricotta mixture.
Repeat the layers once more, finishing with a dollop of the ricotta mixture on top.
Cover the dish with foil and bake for 25 minutes, then remove the foil and bake for an additional 10 minutes until the top is golden and the edges are bubbling.
Let the lasagna rest for 5 minutes before serving to allow the layers to set.