Combine all the marinade ingredients in a small bowl.
Marinade turkey breast: Pat the meat dry with paper towels, then place it in a large ziplock bag. Pour the marinade over the turkey, seal the bag tightly while pressing out excess air, and massage the meat through the bag to coat it thoroughly. Set the sealed bag in a large container (to catch any leaks) and refrigerate for at least 6 hours or, ideally, overnight.
Bring to room temperature: Removethe turkey breast from the bag, reserving the marinade for cooking. Let the meat sit at room temperature for about 20 minutes.
Cook turkey breast:
Sear turkey breast: Sprinkle the breast with salt and pepper on all sides. Heat the oil in the Dutch oven and sear the meat for about 2-3 minutes on each side.
Cook turkey breast: Pour the stock and reserved marinade into the Dutch oven. Cover the pot and simmer over medium-low heat for 1 hour, turning the breast twice to ensure even cooking. Start checking for doneness after 50 minutes if the breast is relatively flat. If the breast is thicker in the center, it may need a little longer. Adjust cooking time as necessary to ensure the meat is fully cooked (Note 3). Use a meat thermometer to verify that the internal temperature has reached 165°F (74°C) at the thickest part.
Rest the meat: Remove the meat from the pot, cover it loosely with foil, and let it rest for about 10 minutes. Meanwhile, make the sauce.
Gravy:
Strain the gravy through a fine-mesh sieve into a small saucepan. Place the saucepan on the stovetop, add the heavy cream, and bring to a gentle simmer.
Thicken the gravy: Whisk together cornstarch and cold water in a small bowl until smooth. Gradually pour the slurry into the simmering gravy, whisking constantly. Let the mixture bubble briefly until it thickens slightly.
Serve: Thickly slice the turkey breast and serve it immediately with the gravy.
Notes
Turkey breast: A little more or less than 2.5 lbs (1.2 kg) is perfectly fine; adjust the cooking time accordingly. You can also use a bone-in turkey breast, but make sure to adjust the cooking time, as bone-in pieces will take longer to cook. The most reliable method to check for doneness is to use a meat thermometer. I always remove the skin when cooking turkey breast in the Dutch oven.
Marinade: You can use another marinade if you like; the flavor will be different but the cooking process remains the same.
Cooking time: The breast will need about 15-20 minutes of cooking time per pound (450 g), but it's best to check with an instant-read thermometer to ensure the meat is safely cooked.