Dissolve yeast in water and let stand 10 minutes. In large bowl, combine flour, sugar and salt. Make a well in flour and add milk, egg, oil and yeast. Combine to make a soft dough. Knead about 10 minutes. Let rise one half hour to one hour.
Brown chopped onion and garlic. In separate pan, brown ground beef. Season with salt, pepper, garlic and onion. Cool meat mixture and remove solidified fat.
Pinch a golf-ball sized piece of dough, flatten with fingers or roll out to 1/8” thickness. Place 2 Tbsp filling in center and bring opposite edges of circle together. Pinch securely. (The traditional shape is a plump center with tapering ends.).
Let piroshkis rise seam side down, 30 minutes. Heat oven to 350. Brush with egg and bake until golden brown (approx 20 min, it depends on the size). The piroshky may also be deep fried.
Ed. Note: I also like to add about 1 cup cheese sauce powder (sometimes found in bulk food stores) to the meat mix. I find it gives a slightly cheesy taste. If I do that, I omit the salt.