In a 10-inch cast iron skillet or other oven-going skillet combine cranberries, cherries, and water. Stir in sugar and the 3 Tbsp. flour. Cook and stir over medium heat until mixture is hot and bubbly. Stir in plums; cover to keep warm.
Preheat oven to 375 degrees F. In a large bowl combine the 1 1/2 cups flour, oats, brown sugar, baking powder, salt, and cream of tartar. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. Make a well in the center of the flour mixture. Add cup milk all at once. Using a fork, stir just until mixture is moistened. Gently fold and press dough against the side of the bowl until it comes together. If the dough sticks to the side of the bowl, sprinkle with a little additional flour.
Lightly flatten small portions of dough with your hands; lay atop filling in skillet. Brush dough lightly with milk; sprinkle with additional brown sugar.
Bake for 35 to 40 minutes or until topping is golden brown and filling is bubbly. (If necessary, cover top loosely with foil during the last 10 minutes to prevent overbrowning.) Let cool for 30 minutes before serving. Serve with cinnamon ice cream.