In a large skillet, melt the butter and oil over medium-high heat. Add the diced onions and sauté until softened and translucent, about 5–7 minutes.

Add the ground beef, breaking it up with a wooden spoon. Season generously with salt and pepper. Cook until the beef changes color and is just cooked through, about 10 minutes. Remove from heat and let it cool slightly.

Ensure your phyllo is fully thawed. Gently unroll the sheets and place them between two damp kitchen towels or plastic wrap. Preheat your oven to 400°F (200°C).

In a small bowl, combine the melted butter, vegetable oil, and mineral water. Whisk briskly until slightly emulsified. Add a pinch of salt.

Lay out one sheet of phyllo dough on a clean surface. Brush it lightly but entirely with the butter-water mixture. Place a second sheet directly on top and brush it again.

Spoon a thin line of the beef filling along the long edge of the doubled phyllo sheet, leaving about an inch border on the sides.

Fold the short sides over the filling. Starting from the long edge with the filling, roll the phyllo into a long, thin rope.

Lightly oil a 9-inch round cake pan. Coil the first roll around the center of the pan, forming a tight spiral. Continue with more rolls, attaching to the end of the previous one, spiraling outwards until the pan is full.

Cover the top with any remaining butter-water mixture. Prick gently with a fork to allow steam to escape. Bake for 40–50 minutes, or until deeply golden brown and sounds hollow when tapped.

Let the pie rest for 10 minutes before slicing into wedges.