Generously season the chicken breasts with kosher salt and ground pepper. Set aside.
Add 1 tbsp extra-virgin olive oil to a large skillet or frying pan over medium-high heat. When the pan is hot, sear the chicken breasts, 3-4 minutes per side, until golden brown on each side. Remove the seared chicken breasts to a plate and set aside.
Turn the crockpot to low heat.
In a medium-sized bowl add the chicken broth, corn starch and heavy cream and whisk well until combined. Add this mixture to the slow cooker and then add the diced onion, minced garlic, dried oregano, dried thyme, chili flakes, salt, oil from the sun-dried tomatoes jar (if using), and the chopped sundried tomatoes. Whisk everything together.
Place the seared chicken breasts into the crockpot and flip them around a few times in the sauce. Spread out in an even layer if possible.
Cover the crockpot and cook on the low setting for 2.5-3.5 hours or until the chicken is cooked through to 165F. Flip the chicken breasts about halfway through the cooking time.
Once the chicken is cooked through, remove the chicken breasts to a plate. Add the freshly grated parmesan cheese to the sauce in the crockpot and stir until the cheese is fully combined into the sauce. Next, stir in the fresh chopped basil and, if desired, stir in the spinach and put the chicken breasts back in. Place the lid back on for 5 minutes and the spinach has wilted (if using).
Give the sauce one more stir, and then serve the chicken topped with the sauce and your choice of sides. Top with more fresh basil if desired.