1.Preheat oven to 180°C (160°C fan-forced). Grease 23cm-square slab pan; line base and two sides with baking paper, extend paper 2cm above edges.
2.In a large frying pan, heat oil and butter on medium heat, add zucchini, leek and garlic; cook, stirring, for 2-3 minutes, or until leek is soft. Transfer to a medium bowl; cool.
3.Add egg, cream, cheeses and mint to bowl, stir to combine; pour into pan. Sprinkle with combined polenta and extra cheeses.
4.Bake for about 40 minutes, or until set. Cool in pan.
5.Cut frittata into 25 pieces. Serve.
Frittata can be made a day ahead; keep, covered, in refrigerator.