Leftover Prime Rib with Mushrooms and Mashed Potatoes

Melt butter in large skillet over medium heat. Add water and minced garlic; stir for 1 minute. Add mushrooms, cover, and cook, stirring occasionally, until mushrooms are tender, about 5 minutes. Add prime rib and mashed potatoes and turn heat to low. Cook, stirring occasionally, until everything is heated through, 5 to 7 minutes. Season with salt and pepper.