Preheat oven to 400 degrees. Melt butter in a large skillet over medium heat. Add shallot and thyme; cook until translucent, about 2 minutes. Add mushrooms; saute until they release liquid and are soft, 3 to 5 minutes. Season with a pinch of salt and pepper. Add Madeira, raise heat to high, and saute until liquid is cooked away, 3 to 5 minutes. Add gravy; cook until thick enough to coat the back of a spoon.
Meanwhile, slice turkey into about 1/4-inch-thick slices, and add to the gravy. Heat until turkey is hot, about 1 minute. Toast bread under the broiler on both sides until golden but still soft.
Place toast on serving plates, spoon 1 or 2 tablespoons gravy over toast, and scatter watercress on top. Arrange turkey over toast, and spoon mushrooms and gravy over meat. Serve warm.
My leftover turkey gravy from Thanksgiving was light, not dark, so I used white wine instead of Madeira. No shallots in the house so I sauteed some finely chopped onion. Even with those changes it was still delicious.
Wow! Really delicious and easy. I made this for my family for Christmas eve. They enjoyed it. I don't eat meat, so I left out the turkey in my portion and really enjoyed it too! It reheats well. Yumm!