Heat 2 tbs oil in a non-stick frypan over medium heat. Add breadcrumbs and cook, stirring, for 5 minutes or until golden. Set aside until needed.
Preheat oven to 180°C.
Heat 2 tbs olive oil in a frypan over low heat and add leek and 1/2 tsp salt flakes. Cook, stirring occasionally, for 20 minutes or until leek is very soft. Add garlic and stir for 2 minutes until softened.
Place grated courgette in a clean tea towel and squeeze out excess liquid over sink. Combine in a bowl with cooled leek mixture, feta, chilli, lemon zest and egg. Lightly season. Spread into a 22cm baking dish.
Toss sliced courgette with remaining 1 tbs oil and layer on top. Scatter with mozzarella. Bake for 30-35 minutes or until golden.
For the caper berry crush, combine all ingredients in a bowl. Spoon over gratin and scatter with breadcrumbs to serve.