Wash and chop mushrooms or use pre-sliced. Sautee in butter until moisture is removed.
While mushrooms are cooking, chop scallions, grate cheese and drain tuna
Place one (uncooked) pie crust in a deep-dish pie plate. Top with tuna, cheese, sautéed mushrooms and scallions.
Beat together eggs, milk and seasonings and pour on top of tuna mixture. (You can add a sprinkle more of cheese on top if you like.) Bake at 350 degrees for 45 minutes. You can protect the crust edges with foil, but I have found that I don't need to. Let cool 10 minutes before cutting.