Preheat oven to 400F/204C. Grease a baking sheet or line with parchment paper. In a large bowl, combine flour, baking powder and salt. Mix well to combine.
Add shortening and cut in with a pastry blender or 2 knives. You can also pulse it in with a food processor. The mixture should resemble coarse oats.
Make a well in the centre and add cooked oatmeal. Add half of the milk and stir to form a soft dough, adding more milk as necessary. You may need to add some additional milk. Knead a few times until the dough is well combined. Do not over knead.
On a lightly floured board, pat or roll dough out to desired thickness. Cut with biscuit cutter and transfer to prepared baking sheet. For a dozen biscuits, roll dough to half inch thickness. Brush tops generously with milk.
Bake 12-15 minutes or until lightly golden. Serve warm.