Preheat your oven to 350°F (175°C) to get it ready for baking the casserole later. Meanwhile, set a pot of water on the stove and bring it to a boil. Stir in a generous pinch of salt to season your pasta as it cooks.

Once the water is boiling, add in the cheese-filled tortellini and cook according to the package instructions, usually 5 to 10 minutes, until they are tender. Drain the cooked tortellini and set aside. I always recommend giving the tortellini a little toss with a touch of olive oil or butter (optional) to prevent sticking, especially if you’re not assembling immediately.

In a large skillet, melt 1 tablespoon of salted butter over medium-low heat. Sprinkle in 1 tablespoon of all-purpose flour and whisk them together to create a smooth paste (roux). Gradually pour in 1 cup of milk, whisking continuously to avoid lumps and form a smooth, thin white sauce. Add 1/4 cup grated parmesan cheese and whisk until it's fully incorporated and the sauce is creamy.

To the cheese sauce in the skillet, add the chopped cooked chicken, fresh spinach, drained tortellini (from Step 2), and 1/4 cup of the shredded mozzarella cheese. Stir everything together gently until the spinach begins to wilt and the cheese is melting into the mixture.

Butter an 8x8-inch casserole dish using softened butter to ensure easy release and prevent sticking. Scoop the prepared chicken tortellini mixture (from Step 4) evenly into the dish. Sprinkle the remaining 1/4 cup of shredded mozzarella cheese and 1/4 teaspoon of Italian herb blend across the top for a flavorful, golden finish.

Bake the casserole in the preheated oven for 15 minutes until the cheese is melty and the mixture is heated through. Switch the oven to broil and cook for an additional 2–3 minutes until the mozzarella on top is golden brown and bubbly. Keep a close eye during broiling—I like to stand by the oven at this stage to make sure the cheese gets perfectly golden without burning. Allow the casserole to cool for a few minutes before serving.