Preheat oven to 375F. Use a small casserole dish, like the square Emilie Henry I have pictured above, and smooth in the mashed potatoes. Next, evenly layer frozen peas, while still frozen. Spoon the stuffing over top, leaving it uneven and tall in certain areas.
Place the butter cubes into the stuffing valleys and place the casserole into the oven. Cook for about 20-25 minutes, until the butter has melted and the stuffing is brown on the top and the center is completely reheated and hot. This cook time will be much shorter if each layer is made fresh and not chilled.