In a large skillet, sauté onion in olive oil for 3-4 minutes. Add minced meat and sauté for 5-8 minutes over medium heat; with a flat-topped wooden spoon break up the meat lumps into small pieces. Stir in cumin, salt and pepper. Remove from heat and cool slightly.
Mash the potatoes; add to the meat and mix to combine.
Cut the filo pastry sheets into 6 approximately equal squares. Place two squares into the filcan or small coffee cup (or a similar shape) so that the sheet ends overhang above the cup edge.
Fill the pastry with the meat and potato mixture, fold the pastry edges over and brush to seal with the egg, milk and oil mixture.
Turn over the filcan with the small burek onto the baking sheet lined with silicone or parchment paper. Brush with the remaining egg, milk and oil mixture, sprinkle with sesame seeds and bake in the oven at 200°C for about 10 minutes.
Take bureks out of the oven, baste each with the water and oil mixture; return to oven and bake 10 minutes at 180°C, or until golden brown.
Serve hot or slightly warm with dollops of sour cream or plain yoghurt.