To begin, preheat your oven to around 400 degrees. While the oven is heating up, you can begin working on the crumb mixture for the mincemeat squares. Take a large bowl, and add in the flour, oats, brown sugar, and salt. If you would rather keep the squares moderately sweet, use just 3/4 cup of the brown sugar.
Once you have added everything to the bowl, stir it around with a spoon until everything is well combined. Then add in the shortening, and cut it into the mixture, using either a knife or a pastry brush. Do not attempt to blend with a spoon – just keep cutting it in, until it is amalgamated into the dry ingredients, and the entire mixture takes on the texture of coarse crumbs.
Now, take a 13-by-9 inch cookie sheet or pan, and press half of the crumb mixture into this. Then, proceed to spread a layer of mincemeat filling over this, making sure it is even, and as thick as you want it to be – ideally, it should be just a little bit thicker than the crumb layer.
Once the mincemeat layer has been spread on, add on the remainder of the crumb mixture, and spread this out as the top layer. Pat it on firmly, and use the back of a spoon to make sure it is smooth, even, and level.
Finally, slide the pan into the oven, and allow it to bake for around 25 to 30 minutes, or until the topping becomes light brown in colour. Take the pan out of the oven and allow it to cool. Then, cut it into equal squares and serve with whipped or clotted cream.