Cook the tortellini. Bring a large pot of salted water to a boil. Cook the tortellini according to the package directions until just tender. Drain and set aside.
Season the chicken. Toss the chicken pieces with paprika, Italian seasoning, onion powder, salt, and black pepper.
Sear the chicken. Heat the olive oil and 3 tablespoons of butter in a large skillet over medium-high heat. Add the chicken in a single layer and cook for 6–8 minutes, turning occasionally, until golden brown and the internal temperature reaches 165°F. Transfer to a plate.
Prepare the garlic butter. Reduce the heat to medium. Add the minced garlic to the same skillet and cook for 30–60 seconds until fragrant, scraping up the browned bits from the bottom.
Make the Alfredo sauce. Add the remaining 2 tablespoons of butter. Once melted, whisk in the flour and cook for 1 minute. Gradually pour in the chicken broth and heavy cream, whisking until smooth.
Add the cheese. Simmer gently for 3–5 minutes until the sauce begins to thicken. Stir in the Parmesan, 1 cup of mozzarella, Italian seasoning, and black pepper until smooth. Season with salt to taste.
Combine the tortellini. Add the cooked tortellini and optional spinach to the Alfredo sauce. Stir gently until the pasta is coated and the spinach has wilted.
Return the chicken. Add the garlic butter chicken and its accumulated juices to the skillet. Toss gently to combine.
Add the cheesy topping. Sprinkle the remaining mozzarella and Parmesan over the top. Cover the skillet for 2–3 minutes, or until the cheese melts. Alternatively, transfer everything to an oven-safe baking dish and broil briefly until bubbly and lightly golden.
Garnish and serve. Sprinkle with fresh parsley and serve hot while the sauce is creamy and the cheese is melted.
Notes
Avoid overcooking the tortellini: Cook it just until tender because it will continue absorbing sauce.
Use freshly grated cheese: It melts more smoothly than pre-shredded cheese.
Keep the sauce creamy: Use low heat after adding the cheese and avoid boiling vigorously.
Adjust the consistency: Add a splash of warm chicken broth if the sauce becomes too thick.
Add extra flavor: Sun-dried tomatoes or sautéed mushrooms make delicious additions.