Preheat oven to 350°F.
In a small frying pan heat the olive oil and add the onions. Cook on medium heat for about 3-5 minutes until the onions are translucent.
Add the beef and the rest of the ingredients. Cook until the beef is browned.
Remove beef mixture from heat, drain any excess liquid, place in a bowl and chill in the refrigerator. Warm meat with juice will ruin the delicate phyllo and make it soggy.
Take phyllo dough and unroll carefully. Cut in half. Cover with a moist tea towel to avoid drying.
Take 3 sheets of phyllo at a time and place a good amount of meat down the center, leaving about 2" at both top and bottom. Using a pastry brush, brush all around the edges of the dough. Fold in once all around. Fold from the short side up and roll tightly like a cigar. Brush with melted butter and place seam side down on a baking sheet. Bake for about 15 minutes or until the burek are golden brown. Serve with a side of plain yogurt.
Any leftover phyllo can be stored again in the freezer. Make sure you wrap it tightly so it doesn't dry out.