1. In a pan, add the olive oil and once hot, add the leek and onion. Sauté and then add the mushrooms. Let them cook until they release all their liquid. Add the lamb and stir. Add the pepper, corn, and tomato. Sauté for 1-2 minutes, then add the parsley and salt. Stir and allow all the liquids to evaporate. Turn off the heat and add the cheeses. Mix well to ensure the cheese is evenly distributed.
For the Pie:
1. Pour a little olive oil into the base of a rectangular baking dish measuring 24x33cm. Lay one rustic phyllo sheet and drizzle a little olive oil over it. Repeat with 3 more sheets.
2. Add the filling and cover with the remaining 3 phyllo sheets, drizzling a little olive oil on each. Drizzle any remaining olive oil over the surface of the pie. Trim any excess phyllo and fold the edges that extend from the dish inwards. Cut the pie into 12 pieces and sprinkle with sesame seeds. Bake in a preheated oven at 180°C (fan-assisted) for 45 minutes.