2) Arrange the aubergine slices in 1 layer onto a baking sheet. Drizzle with olive oil and roast until they are soft and pliable, about 15 to 20 minutes, turning half way through the cooking time. Reserve the 4 largest rounds and finely chop the remaining aubergine.
3) Heat a large skillet over a medium-high heat. When hot, add the onions and season with salt; cook until soft, about 5 minutes. Put the cooked onion and chopped aubergine in a large bowl. Add the feta, bread crumbs, kasseri, lemon zest, oregano, chilli flakes and garlic. Add 2 to 3 tbsp olive oil and combine well. Taste and adjust the seasoning, if needed.
4) Divide the filling among the 4 reserved aubergine rounds. Roll the aubergine around the filling and arrange on a serving platter with the seam side down.
5) Combine the yogurt and mint together in a small bowl. Garnish each aubergine roll with a dollop of mint yogurt and serve at room temperature.