meat stoneware plate used for dairy

My housekeeper put dairy cooked food (that had cheese) on a meat stoneware plate into the microwave to heat. Is there any way to kosher the plate?
She also used the plastic meat microwave cover to warm the dairy food. What to do with that?
Thank you in advance for your attention toward this matter.
Rozie Matthews

The plastic cover is not an issue. When you say stoneware, what type? Is it smooth and glazed or raw?