Grease 12 jumbo or 24 medium muffin tins generously with butter.
Preheat the oven to 170°C.
Rub the flour and butter together until it resembles fine crumbs and sprinkle about ¾ of this mixture evenly onto the base of the muffin tins.
Press down lightly to form an even layer and sprinkle half of the cheese, onion or spring onion, flaked fish and peppadews, if used, over the crumbs.
Beat together the milk, eggs, mustard powder and salt and ladle over the filling ingredients, dividing it evenly between the muffin tins.
Mix the remaining cheese and crumbs and sprinkle over the liquid.
Bake 20 - 25 minutes for the large or 15 - 20 minutes for the smaller tartlets until set and golden on top.
Serve warm or cool and refrigerate to reheat when required.