Heat oven to 375°F and position a rack in the center.
Pat chicken dry. Season on both sides with 1 tsp salt, 0.5 tsp pepper, paprika, garlic powder, and Italian seasoning.
Warm olive oil and butter in a large oven-safe skillet over medium-high heat. Sear chicken 3–4 minutes per side until deep golden; transfer to a plate.
Lower heat to medium. Cook onion in the same skillet until softened, about 3 minutes. Stir in minced garlic for 30 seconds.
Sprinkle in flour and cook, stirring, 30 seconds. Whisk in broth, then heavy cream; simmer until slightly thickened, 2–3 minutes.
Melt in cream cheese until smooth. Add spinach in batches, tossing until wilted.
Stir in Parmesan, red pepper flakes, lemon juice, and sun-dried tomatoes if using. Season with remaining salt and pepper to taste.
Nestle chicken into the creamed spinach and spoon sauce over the tops. Scatter mozzarella over the chicken.
Bake 8–10 minutes, or until the chicken reaches 165°F and the cheese is melted and bubbly. Rest 5 minutes before serving.