Step 1: Prepare and Season the Chicken
Pat the chicken breasts completely dry with paper towels. This is the most crucial step for a good sear. In a small bowl, combine the Italian seasoning, garlic powder, onion powder, 1 teaspoon salt, and ½ teaspoon black pepper. Sprinkle the mixture evenly over both sides of each chicken breast, pressing gently to adhere.

Step 2: Sear the Chicken
Heat the olive oil in a large (12-inch), heavy-bottomed skillet (cast iron or stainless steel) over medium-high heat. Once the oil is shimmering, add the chicken breasts. Sear without moving for 5-7 minutes, until a deep golden-brown crust forms. Flip and cook for another 5-7 minutes, or until the chicken is cooked through (internal temperature of 165°F / 74°C). Transfer the chicken to a clean plate and tent loosely with foil.

Step 3: Build the Aromatic Base
Reduce the heat to medium. Add the butter to the same skillet. Once melted, add the diced onion or shallot. Cook, stirring occasionally, for 3-4 minutes until softened and translucent. Add the minced garlic and cook for 1 more minute until fragrant, being careful not to let it burn.

Step 4: Deglaze and Create the Sauce Base
Pour in the white wine (or chicken broth) to deglaze the pan. Use a wooden spoon to scrape up all the flavorful browned bits (fond) from the bottom—this is pure flavor. Let it simmer for 2-3 minutes until reduced by about half. Stir in the chicken broth, heavy cream, dried thyme, and nutmeg (if using). Bring the mixture to a gentle simmer.

Step 5: Wilt the Spinach and Thicken the Sauce
Add the fresh spinach to the simmering sauce, one large handful at a time, stirring until each addition wilts before adding the next. This will take 2-3 minutes total. If using cream cheese, add it now and whisk until fully melted and incorporated into the sauce. Let the sauce simmer for 3-4 minutes to thicken slightly.

Step 6: Incorporate the Parmesan
Reduce heat to low. Gradually sprinkle in the grated Parmesan cheese, whisking constantly until the cheese is fully melted and the sauce is smooth. Taste the sauce and adjust seasoning with salt and pepper as needed. The sauce should be rich and coat the back of a spoon.

Step 7: Return Chicken and Finish
Return the seared chicken breasts and any accumulated juices to the skillet, nestling them into the creamy spinach sauce. Spoon some of the sauce over the top of each breast. Let everything heat through for 3-4 minutes to allow the flavors to meld.

Step 8: Garnish and Serve
Remove from heat. Garnish generously with fresh chopped parsley or basil and a pinch of red pepper flakes for a touch of heat if desired. Serve immediately directly from the skillet, with lemon wedges on the side for a bright, fresh squeeze over the top.