Steam will help a leftover egg breakfast casserole that seems dried out. While the leftovers sit covered on the counter to take the chill off, sprinkle water droplets over the top, approximately 1-2 tablespoons. The most crucial step is to let the breakfast casserole warm up on the counter to avoid prolonged heating in the oven.  Keep the casserole tightly covered with foil and put it in a preheated 350°F oven. Carefully check at the 15 minutes mark. If all the water has diffused, scatter some butter pats or drizzle with a bit of melted butter for the remaining five minutes. Let it rest with the foil for another 5 minutes out of the oven.