Step 1: Cook the Tortellini
Bring a large pot of generously salted water to a boil. Cook the tortellini according to package directions for al dente. Typically, fresh tortellini cooks in 3-4 minutes, frozen in 5-7 minutes. Before draining, carefully reserve ½ cup of the starchy pasta water. Drain the tortellini and set aside. Do not rinse.

Step 2: Prepare and Sear the Chicken
While the pasta water heats, pat the chicken dry. Combine salt, black pepper, Italian seasoning, and garlic powder. Rub this mixture evenly over all sides of the chicken. In a large, deep skillet or Dutch oven, heat the olive oil and 2 tablespoons of butter over medium-high heat. Once hot, add the chicken and cook for 5-7 minutes per side, or until deeply golden brown and cooked through (internal temperature of 165°F / 74°C). Transfer to a clean plate, tent with foil, and let rest.

Step 3: Build the Garlic-Infused Roux
In the same skillet, reduce heat to medium. Add the remaining 4 tablespoons of butter. Once melted, add the minced garlic and cook for 1 minute until fragrant but not browned. Sprinkle the flour over the garlic butter and whisk constantly for 1-2 minutes to form a pale golden roux, cooking out the raw flour taste.

Step 4: Create the Cream Sauce Base
Slowly and gradually, while whisking constantly, pour in the chicken broth. Whisk vigorously to fully incorporate the roux and prevent lumps. Allow the mixture to simmer for 2-3 minutes until slightly thickened. Then, slowly whisk in the heavy cream. Bring the sauce to a gentle simmer (do not boil vigorously) and let it cook for 4-5 minutes, stirring occasionally, until it thickens enough to coat the back of a spoon.

Step 5: Incorporate the Parmesan and Season
Reduce heat to low. Gradually sprinkle in the grated Parmesan cheese, whisking constantly until the cheese is fully melted and the sauce is smooth. Stir in the nutmeg. Taste the sauce—this is crucial. Season carefully with salt (remember, Parmesan is salty) and white pepper. The sauce should be highly seasoned to carry the entire dish.

Step 6: Slice Chicken and Combine
Slice the rested chicken into ½-inch thick strips or bite-sized pieces. Add the cooked, drained tortellini and the sliced chicken to the sauce in the skillet. Gently toss to combine, ensuring everything is heated through and coated in sauce. If the sauce seems too thick, add the reserved pasta water a few tablespoons at a time until it reaches a lush, flowing consistency.

Step 7: Final Touches and Garnish
If using, add the fresh spinach and stir until just wilted, about 1 minute. Remove the skillet from heat. Stir in the fresh lemon juice, half of the chopped parsley, and half of the basil. Serve immediately, garnished with the remaining fresh herbs and an additional generous sprinkling of freshly grated Parmesan cheese.