Preheat your oven to 400ºF (200ºC). Lay the sliced boneless, skinless chicken breasts flat in a large baking dish. Season the chicken breasts evenly with 1/4 teaspoon salt, 1/4 teaspoon black pepper, and half of the minced garlic. Make sure the seasonings are distributed well over the surface of the chicken.

In a medium bowl, combine the thawed and drained spinach, 3 oz cream cheese at room temperature, and 1/3 cup heavy cream. Add the remaining minced garlic, 1/2 tsp onion powder, 1/4 tsp chili powder, 1 tsp Italian seasoning, a pinch of salt, and a pinch of black pepper (take these pinches from your existing measurements for nuanced flavor). Mix thoroughly until the fillings are creamy and well blended.

Stir 1/3 cup grated parmesan and half of the shredded mozzarella into the creamy spinach mixture from Step 2. Once mixed, spread the filling evenly over the seasoned chicken breasts from Step 1 in the baking dish. Top with the chopped sun-dried tomatoes, then drizzle 1 tablespoon of reserved oil from the sun-dried tomatoes evenly over everything. Finally, sprinkle the remaining mozzarella cheese on top for a nice golden finish when baked. I like to reserve some extra mozzarella to sprinkle just before baking for an extra cheesy crust.

Cover the baking dish loosely with foil and place it in the preheated oven. Bake for 20 minutes. Then, remove the foil and continue baking for another 15 minutes, or until the chicken is fully cooked through and the casserole is golden and bubbly. The chicken should reach an internal temperature of 165ºF (71ºC) for safe consumption.

Remove the casserole from the oven and allow it to cool for a few minutes. Garnish with freshly chopped parsley just before serving. I like to add a squeeze of fresh lemon juice for brightness if I have one on hand. Serve the Tuscan chicken casserole hot, perhaps with cauliflower rice or zucchini noodles for a lighter side.