Preheat over to 375 degrees Farenheit
Wash and pit plums, slice into pieces lengthwise.
Add butter and plums to a cast-iron or oven-proof skillet along with white sugar (reserve one tablespoon to sprinkle on the topping), lemon juice, cinnamon, salt and freshly grated nutmeg. Combine well. If you don't have a cast-iron skillet just use a regular skillet to cook the fruit mixture and then transfer it when fully cooked to an 8 x8 inch baking dish. Cook on medium-high heat until the fruit mixture comes to a boil, Reduce heat and simmer for 7 minutes.
Meanwhile, mix together cornstarch and water until smooth. Stir into plum mixture and all it to cook and thicken for five minutes.
Combine the masa flour (or whatever GF flour you are using), baking powder and brown sugar in a bowl.
Cut in shortening using two knives or pastry cutter.
Beat egg in a bowl with a fork and combine with milk.
Add all at once to dry ingredients.
Form the dough into a ball and knead lightly. Place on a floured surface and pat or roll into a rectangle about 1/2 inch thick. Using a small glass or cookie cutter, cut the biscuit dough into rounds.
Place the rounds biscuits on top of the plum mixture in the skillet or baking dish.
Sprinkle a tablespoon of white sugar over top.
Place into the pre-heated oven and bake for 30-35 minutes, or until plum filling is bubbling and biscuits are a golden brown.
Serve warm with vanilla ice-cream or heavy cream.