Preheat your oven to 375°F (190°C).
In a small skillet over medium heat, brown the ground beef with the diced onion and minced garlic until the meat is fully cooked.
Stir in the fresh baby spinach and half of the marinara sauce, simmering for 2 minutes until the spinach is wilted and the flavors meld.
In a small bowl, combine the ricotta cheese, whisked egg, dried oregano, sea salt, and black pepper, mixing until smooth and creamy.
Boil the whole wheat lasagna sheets in a pot of salted water according to package directions until al dente, then drain and pat dry.
In a small individual-sized oven-safe dish, spread a thin layer of the remaining marinara sauce on the bottom.
Layer one lasagna sheet (cut to fit), followed by half of the meat mixture, half of the ricotta mixture, and a sprinkle of mozzarella.
Repeat the layer with the second lasagna sheet, the remaining meat mixture, and the remaining ricotta mixture.
Finish with the remaining marinara sauce and a generous layer of mozzarella cheese.
Cover the dish with foil and bake for 20 minutes, then remove the foil and bake for an additional 10 minutes until the top is bubbly and golden.