Preheat the oven to 400 degrees F. Place the butter in a 9-inch pie plate and heat in the oven for 3 to 4 minutes until melted. In a bowl, whisk the milk, flour, eggs and salt for 3 minutes, or until smooth. Pour the batter into the prepared dish. Bake 20 to 25 minutes or until brown.
FILLING & FINISHING
Squeeze the excess moisture from the thawed spinach. Melt the butter in a large skillet. Add the mushrooms and onion to the butter and cook over medium heat for 5 minutes, or until the mushrooms are tender, stirring often. Stir in the spinach and ham.
Cook 3 to 4 minutes longer, or until the filling is heated through, stirring occasionally. Stir in 1 cup of the cheese. Spoon the spinach mixture into the center of the hot pancake and sprinkle with the remaining ½ cup cheese. Place it back into the oven for a few minutes, just until the cheese is melted.