Put 3 cups flour in a pot, make a well in the middle for the egg and crack the egg into this well. Add some salt on it and add lukewarm water gradually. Knead the dough until it is tough and elastic, then put the dough in the fridge for some time. While the dough is resting in the fridge, start preparing the filling. Grind one onion in the ground beef, add salt and black pepper and mix it. Take the dough out of the fridge and divide it into equal lumps and start rolling these lumps 2 mm thin. After rolling, cut the dough in small squares with a knife and put the filling on these small dough squares. Then, put mantıs in boiling water and cook for 15-20 minutes. Take boiled mantıs from water and put them on a service plate, pour yogurt with garlic on them.