Heat olive oil and butter in a large deep skillet or pot over medium heat. Add chicken, season lightly with salt and pepper, and cook for 4–5 minutes until lightly golden.
Add garlic and sun-dried tomatoes. Cook for 30 seconds, just until fragrant.
Stir in uncooked penne and pour in chicken broth. Bring to a gentle simmer.
Reduce heat to medium-low and cook, stirring occasionally, for 10–12 minutes, until pasta is al dente and most of the liquid is absorbed.
Stir in heavy cream, Italian seasoning, Parmesan cheese, and red pepper flakes if using.
Add spinach and stir until wilted and the sauce is creamy.
Simmer gently for 2–3 minutes, then remove from heat. Taste and adjust seasoning.
Garnish with fresh herbs and serve warm.