Melt 4 tablespoons butter in a large frying pan over medium heat. Add ground beef and sauté for 5 minutes.
Add onions and continue to sauté for an additional 5 minutes.
Add peppers and tomatoes, sautéing for 10 minutes. Remove from heat and set aside.
Preheat oven to 350°F.
Place remaining butter in a heat proof bowl and microwave until melted.
Beat egg and add to the bowl of butter along with the milk, whisking until combined.
Prepare a 12 x 15 baking pan with oil or butter.
Lay one sheet of pastry dough on bottom of pan, cutting the edges and sides to fit the pan if necessary. Brush butter mixture over dough and repeat by layering the dough and butter until half of the phyllo dough is used.
Spread the meat mixture over the dough evenly.
Continue to layer the remaining dough and butter mixture until all of the dough is used. Sprinkle the top with sesame seeds. Cut the Börek into 16 even squares.
Bake for 30 minutes or until the top is lightly browned. Remove from oven and let rest for 10 minutes before serving.
Recipe by Suheyla Kerkinni for www.culinarybutterfly.com