In a large, heavy-bottomed skillet, heat the olive oil over medium-high heat until shimmering.
Add the ground chicken, using a wooden spoon to break it into very small crumbles, and cook undisturbed for 5 minutes to develop a deep golden crust.
Stir in the finely diced yellow onion and minced garlic, reducing the heat to medium and cooking for 8-10 minutes until the onions are caramelized and sweet.
Add the sea salt, black pepper, and dried oregano, followed by the marinara sauce, and bring the mixture to a gentle simmer for 15 minutes to concentrate the flavors.
While the sauce simmers, break the whole wheat lasagna noodles into uniform 2-inch pieces.
Carefully tuck the noodle pieces into the sauce, ensuring they are submerged and layered evenly throughout the skillet.
Cover tightly and simmer on low for 20-25 minutes, or until the noodles are perfectly al dente and have absorbed the savory sauce.
Fold in the fresh spinach in batches until completely wilted, then dollop the ricotta cheese across the surface.
Sprinkle the shredded mozzarella over the top, cover one last time, and let sit for 5 minutes off the heat until the cheese is gooey and melted.