Pat the chicken breasts dry with paper towels. If they are thick, pound them to an even thickness of about ½ inch so they cook evenly. In a small bowl, combine the garlic powder, onion powder, paprika, salt, and black pepper. Rub the seasoning mixture evenly over both sides of the chicken. Let the chicken rest for 15 to 20 minutes so it absorbs the flavors.

Heat 2 tablespoons of butter in a large skillet over medium heat. Once the butter has melted and is lightly bubbling, add the chicken. Cook for about 4 to 5 minutes per side, or until golden brown and the internal temperature reaches 165°F (74°C). Transfer the chicken to a cutting board and let it rest for a few minutes before slicing it into thin strips or bite-sized pieces.

Using the same skillet, melt the remaining 2 tablespoons of butter. Add the finely chopped onion and cook over medium heat for 4 to 5 minutes, stirring occasionally, until it becomes soft and translucent. This creates a flavorful base for the sauce.

Stir the tomato paste into the onions and cook for 1 to 2 minutes. This helps deepen the tomato flavor and removes any raw taste. Gradually pour in the chicken stock while stirring to loosen the tomato paste. Add the heavy cream and continue stirring until everything is smooth and well combined.

Bring the sauce to a gentle simmer and cook for 3 to 5 minutes until it begins to thicken slightly. Stir in the chopped sun-dried tomatoes, followed by the baby spinach. Cook for another 1 to 2 minutes until the spinach has wilted. Add 1 cup of shredded mozzarella cheese and stir continuously until it has completely melted into the sauce, creating a rich, creamy consistency.

Preheat your oven broiler. Slice the sourdough loaf in half lengthwise. If desired, lightly toast the cut sides of the bread for 3 to 4 minutes before adding the toppings. This helps keep the bread crisp while supporting the creamy filling.

Spread the creamy mozzarella sauce evenly over both halves of the sourdough. Arrange the sliced chicken evenly over the sauce, making sure each piece gets a generous amount. Sprinkle 1 to 1½ cups of shredded mozzarella over the top, covering the chicken completely.

Place the assembled bread on a baking tray and broil for 3 to 5 minutes, keeping a close eye on it. The cheese should melt completely, become bubbly, and develop light golden brown spots. Remove it from the oven as soon as the cheese is perfectly melted to prevent the bread from burning.

Allow the bread to cool for 2 to 3 minutes before slicing. Cut into thick pieces using a serrated knife and serve warm while the cheese is still gooey. Pair it with a crisp green salad, roasted vegetables, or a bowl of soup for a complete meal.