In a large skillet, heat a splash of oil over medium heat. Add chopped onion, sautéing until soft and translucent, about 4-5 minutes. Stir in ground beef, cooking until browned. Fold in rice, tomato sauce, and oregano, along with salt and pepper. Pour in enough water to moisten, cover, and simmer for 10 minutes.

Preheat your oven to 180°C (350°F). Brush a 9x13-inch baking dish with melted butter. Layer five sheets of phyllo, brushing each with butter for crispiness.

Carefully spoon the meat mixture onto the phyllo layers, spreading it evenly. Leave space around the edges for folding.

Cover the filling with five more phyllo sheets, repeating the brushing process. Tuck edges of the phyllo around the filling into the dish.

Place the Kreatopita in the preheated oven and bake for 40-45 minutes until the top is golden brown. Let cool for 10 minutes before slicing.