Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish with cooking spray or butter.
Cook the tortellini according to package instructions until al dente. Drain and set aside.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté for about 3-4 minutes until softened. Add the minced garlic and cook for an additional 1 minute until fragrant.
Stir in the heavy cream, Parmesan cheese, Italian seasoning, salt, and pepper. Cook for 2-3 minutes until the sauce is heated through and slightly thickened.
Add the cooked tortellini and spinach to the skillet, stirring gently to combine everything evenly.
Pour the tortellini mixture into the prepared baking dish. Sprinkle the shredded mozzarella cheese evenly over the top.
In a small bowl, combine the breadcrumbs and melted butter. Sprinkle this mixture over the cheese layer.
Bake in the preheated oven for 25-30 minutes, or until the top is golden and bubbly.
Remove from the oven and let it cool for a few minutes before serving. Garnish with fresh parsley.