Mix 1 1/4 cups of flour, the cornmeal, 1/2 cup of sugar, and 1/2 teaspoon of salt in a bowl. Add the cup of margarine and mix with a fork or pastry blender until it forms clumps. Add the water and mix just until it comes together. It should be softer than but similar to cookie dough. Divide the dough into quarters and then thirds, making 12 dough pieces. Form each piece into a ball with your hands, and then press down to make a small patty. Continue flattening the patty until you have a 1/4"-1/8" thick, 3"-4" circle. Place the flattened dough into one of the cups in a twelve-cup non-stick muffin pan. It does not have to fit perfectly, and probably won't. Press out the sides and any possible overlap of the dough to form a tart crust in the muffin-cup. It does not have to overlap the edge of the muffin-cup, just come to the top of it. Repeat this for all twelve muffin-cups. Transfer the pan to the freezer or refrigerator.
Mix the 1/2 cup of flour, 1/2 cup of sugar, 1/2 teaspoons of salt, and the cinnamon in a bowl. Add the diced apples and toss to coat them. Remove the apples from the flour mixture and set aside. Mix the oats into the flour mixture. Add the 1/4 cup of margarine to the oat mixture and mix until it is a crumbly, grainy texture (somewhat loose). Set this aside.
Preheat the oven to 350 degrees. Get your muffin pan out of the freezer and spoon the coated apples into each tart-cup. Since the apples shrink some during baking, make sure to heap them slightly above the level of the crust. (If you have extra, bake the apples in a small oven-safe bowl or dish - they're great to eat without the fat of the crust!) Spoon the oat mixture over top of the apples and crust, covering them almost completely.
Bake for 45 minutes at 350 degrees. Let cool for ten to twenty minutes before removing from the pan. Heavy cream or milk can be spooned over the tarts at serving if desired. Nutmeg can also be substituted for the cinnamon if preferred. Enjoy!