Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch casserole dish.

Cook the pasta in a large pot of salted water according to package directions until al dente. Drain well and set aside.

While the pasta cooks, heat olive oil in a large skillet or Dutch oven over medium heat. Add the chopped onion and cook until softened, about 5-6 minutes. Add the minced garlic and cook for another minute until fragrant.

Add the fresh spinach to the skillet in batches, stirring until it wilts down completely. Season with a pinch of salt and pepper. Remove the spinach and onion mixture from the skillet and set aside.

In the same skillet, melt 4 tbsp of butter over medium heat. Whisk in the all-purpose flour and cook for 1 minute, stirring constantly, to create a roux.

Slowly whisk in the chicken broth until smooth. Then, gradually whisk in the heavy cream. Bring the mixture to a simmer and cook for 2-3 minutes, until it starts to thicken.

Reduce the heat to low. Add the cubed cream cheese and whisk until it has completely melted into the sauce. Stir in the grated Parmesan cheese, dried oregano, salt, and pepper. Continue stirring until the sauce is smooth and creamy. Taste and adjust seasoning if necessary.

To the skillet with the sauce, add the cooked pasta, shredded chicken, and the cooked spinach-onion mixture. Stir gently until everything is well-coated in the sauce.

Pour the mixture into the prepared casserole dish and spread it into an even layer.

In a small bowl, combine the panko breadcrumbs with the 2 tbsp of melted butter. Sprinkle the shredded mozzarella cheese evenly over the casserole, followed by the buttered panko breadcrumbs.

Bake uncovered for 25-30 minutes, or until the sauce is bubbly and the topping is golden brown.

Let the casserole rest for 5-10 minutes before serving. This allows it to set and makes it easier to serve.