Preheat oven to 375°F (190°C). Cook shells until al dente; drain and cool.
Heat oil in a skillet, sauté onion 3–4 minutes, add garlic and minced chicken; cook until no pink remains.
Stir in spinach until wilted, season with Italian seasoning, salt, and pepper; remove from heat.
Mix chicken mixture with ricotta, 1/2 cup mozzarella, and 1/4 cup Parmesan.
Spoon filling into shells and place in a baking dish spread with 1 cup marinara; top shells with remaining sauce and cheeses.
Bake 20–25 minutes until bubbly and golden on top; let rest 5 minutes before serving.