Preheat the oven to 350°F (180°C) or 160°C if using a fan oven.
In a medium bowl or a jug, combine half and half with garlic, dijon mustard, cornstarch, oregano, thyme, and smoked paprika. Whisk until well combined.
Add diced sundried tomatoes and parmesan cheese and stir to combine. Set aside.
To a 9x13 inch casserole dish, add the chicken and season with salt and pepper.
Pour the sauce around the chicken, and bake in the oven for 15-20 minutes (exact time depends on the thickness of the chicken). The chicken is done when the internal temperature of the thickest part reaches 165°F (74°C). Switch off the oven and remove the pan.
Sprinkle the spinach all over the sauce, and carefully stir most of it in the sauce with a small spatula. Return to the warm oven (no need to switch it back on), and let the casserole sit for 5-10 minutes or until the spinach starts to wilt. Serve warm over pasta, rice, or cauliflower rice.