Remove the giblets and neck from the turkey breast if necessary.
Rinse the turkey breast under cold water and pat dry.
Place the turkey breast in the cast iron roasting pan.
Pour the water into the bottom of the roasting pan.
Start at the neck cavity and insert your fingers in between the skin and the breast so you can separate the skin, but do not remove it.
Place the lemon quarters and half of the fresh herbs into the area between the skin and the top of the turkey breast.
If there are too many herbs to fit into this area without tearing the skin, put the rest of them into the cavity.
Now tuck the skin flap into the cavity to close the herbs, lemons and cavity up.
Pour the cajun butter over the top of the turkey, then sprinkle the salt and pepper on top.
Place the lid on the cast iron roasting pan and place it on the bottom rack of the oven.
Turn the oven to 400 degrees Fahrenheit.
Once the oven is preheated, cook the turkey breast for 15 to 20 minutes per pound. The exact cooking time per pound will be on the package your turkey comes wrapped in. The time will also vary if you decide to make a stuffed turkey.
Set the timer, making sure to baste the turkey once an hour using the juices that fall into the bottom of the roasting pan.
Once the timer goes off, use a meat thermometer into the area between the innermost part of the thigh and the wing to check the internal temperature of the turkey. The internal temperature should be 165 degrees Fahrenheit.
If the internal temperature is not high enough, replace the lid then return the turkey to the oven and continue cooking it.
If the internal temperature is correct, put the lid back on the roasting pan and allow the turkey to rest for 20 minutes before removing it from the roasting pan and carving it.
Once you are ready to carve the turkey breast, remove it from the cast iron roasting pan and carve it.