Begin by patting the boneless, skinless chicken dry with paper towels. Season both sides generously with salt, pepper, Italian seasoning, and paprika.

Heat a tablespoon of olive oil in a large skillet over medium heat. Add the seasoned chicken and sear for 1-2 minutes on each side until golden brown.

In the same skillet, add another drizzle of olive oil along with a tablespoon of butter. Sauté sliced mushrooms for 5-6 minutes until tender, then add minced garlic.

Pour in chicken broth and scrape the bottom of the skillet. Stir in heavy cream, optional Dijon mustard, red pepper flakes, and Parmesan cheese. Simmer for 3-5 minutes until thickened.

Stir in fresh baby spinach and cook until wilted, about 2 minutes. If using frozen spinach, ensure it's well-drained before adding.

Nestle the seared chicken back into the sauce, then sprinkle mozzarella and remaining Parmesan atop. Preheat oven to 400°F (200°C) and bake for 18-22 minutes until chicken is cooked through.

Allow the casserole to rest for 5 minutes before serving, garnished with parsley or chives.