In a skillet, sauté diced onions in a splash of oil over medium-high heat until golden. Add diced green peppers and tomatoes, cooking for an additional 5 minutes. Incorporate ground beef, cooking until browned and juices are absorbed. Allow the filling to cool completely.

In a separate bowl, whisk together the eggs, milk, and oil until well combined. This mixture will help achieve a beautiful finish on your borek.

Carefully cut each Turkish pastry leaf into quarters; a sharp knife will make the job easier.

Take a pastry triangle, brush it with the egg wash, and place 2-3 tablespoons of your filling on one side. Gently fold the edges and roll towards the pointed edge to create a neat shape.

Arrange the borek on a greased baking sheet, brushing the tops with the remaining liquid mixture. Sprinkle with nigella seeds.

Bake in a preheated oven at 350°F (180°C) for about 25-30 minutes, or until golden brown and crispy.