Season the chicken. Pat the chicken breasts dry and season both sides evenly with Italian seasoning, kosher salt, and black pepper.
Sear the chicken. Heat 2 tablespoons of olive oil in a large skillet over medium heat. Add the chicken breasts and cook for 3 to 4 minutes per side, or until golden brown. Transfer the chicken to a plate and set aside.
Cook the onion. Add the remaining tablespoon of olive oil to the same skillet. Add the chopped onion and sauté for 2 to 3 minutes, until softened.
Add the aromatics. Stir in the sun-dried tomatoes and minced garlic. Cook for about 1 minute, stirring frequently, until fragrant.
Add the spinach. Add the baby spinach and cook for about 1 minute, just until it begins to wilt.
Make the sauce. Pour in the heavy cream and add the freshly grated Parmesan. Stir well and bring the sauce to a gentle simmer.
Finish the chicken. Return the seared chicken and any accumulated juices to the skillet. Let everything simmer gently for about 5 minutes, or until the chicken is fully cooked and the sauce has thickened slightly.
Serve. Spoon the creamy sauce, spinach, and sun-dried tomatoes over the chicken and serve immediately with your favorite side.