Preheat the oven to 350°F (180°C), or 160°C for a fan oven.
Prepare the sauce: In a medium bowl or jug, whisk together: Half and half, Garlic, Dijon mustard, Cornstarch, Oregano, Thyme, Smoked paprika. Stir until fully combined.
Stir in the sun-dried tomatoes and parmesan cheese. Set the sauce aside.
Prepare the chicken: Season the chicken breasts with salt and pepper. Place them in a 9×13-inch casserole dish. Pour the sauce over and around the chicken.
Bake the chicken: Place the casserole dish in the oven and bake for 15-20 minutes. Use a thermometer to check the internal temperature. The chicken should reach 165°F (74°C). Once cooked, turn off the oven and remove the pan.
Add spinach: Sprinkle chopped spinach over the sauce. Gently stir it into the sauce using a small spatula. Return the pan to the warm oven (no need to turn it back on) for 5-10 minutes, or until the spinach wilts.
Serve: Serve the chicken and sauce over pasta, rice, or cauliflower rice.