In a large skillet, heat a splash of oil over medium heat. Add the chopped onion, sautéing until it becomes soft and translucent, about 5 minutes. Stir in the ground beef, cooking until it browns completely, approximately 7-10 minutes. Incorporate the uncooked rice, tomato sauce, oregano, salt, pepper, and water, stirring well. Let this mixture simmer on low heat for 10 minutes until the rice absorbs flavors and becomes slightly tender.

Preheat your oven to 180°C (350°F) while preparing the baking dish. Brush a medium-sized baking dish with melted butter or oil. Carefully lay down 5 sheets of phyllo dough, brushing each layer with butter or oil. Keep the remaining phyllo covered with a damp towel.

Spread the savory beef and rice filling evenly over the prepared phyllo layers in the baking dish. Cover it with another 5 sheets of phyllo, brushing each with melted butter or oil. Ensure the final layer is well-brushed.

Place the assembled Kreatopita in the preheated oven and bake for 40-45 minutes until golden and crisp. Let it cool for about 5-10 minutes before slicing into squares.