Preheat your oven to 375°F (190°C).
Bring a pot of salted water to a boil and cook the lasagna noodles according to package directions until al dente, then drain and set aside.
In a skillet over medium heat, add the olive oil, diced onion, and minced garlic, sautéing until translucent.
Add the ground turkey sausage to the skillet, breaking it up with a spatula, and cook until browned and fully cooked through.
Stir in the fresh baby spinach and cook for 1 minute until wilted, then remove from heat.
In a small bowl, mix the ricotta cheese with the dried oregano, sea salt, and black pepper.
In a small individual baking dish, spread a thin layer of marinara sauce on the bottom.
Layer with a piece of lasagna noodle (cut to fit), followed by half of the sausage and spinach mixture, half of the ricotta mixture, and a spoonful of marinara.
Repeat the layer once more, ending with a final piece of noodle, the remaining marinara, mozzarella, and parmesan cheese.
Cover with foil and bake for 15 minutes, then remove the foil and bake for another 5-10 minutes until the cheese is bubbly and golden brown.
Let the lasagna rest for 5 minutes before serving.