In a large bowl, combine the mashed potatoes, drained tuna, chopped parsley, finely chopped onion, capers (if using), ground cumin, salt, and black pepper. Mix well to combine.
Lay out a sheet of brick pastry (or phyllo dough) on a flat surface. If using phyllo dough, you may need to brush it lightly with oil or melted butter and layer two sheets together to prevent tearing.
Place about 2 tablespoons of the filling in the center of the pastry sheet.
Make a small well in the center of the filling and carefully crack an egg into the well.
Fold the edges of the pastry over the filling to form a triangular or square packet, ensuring the egg is well enclosed.
In a large frying pan, heat vegetable oil over medium heat until hot.
Carefully place the brik packets into the hot oil, seam side down.
Fry for about 2-3 minutes per side, or until the pastry is golden brown and crispy and the egg inside is cooked to your desired level of doneness.
Remove the brik from the oil and drain on paper towels.
Enjoy your meal!