Use a 24cm ceramic fluted quiche dish; grease the dish. Preheat oven to 210°C.
Heat 2 tbsp olive oil in a frying pan. Sauté the onion and garlic and cook until they are both transparent. Remove from heat and let mixture cool.
Peel potatoes and grate finely. Discard ends of zucchini, grate the flesh coarsely. Combine onion mixture, grated vegetables and the lemon zest in a bowl. Add chopped herbs and dried oregano.
Stir the eggs, cornflour, 1 tbsp olive oil, the sour cream and cheese into the vegetable mixture. Season well with salt and pepper.
Pour vegetable mixture into the flan dish; smooth top.
Sprinkle with Parmesan and hazelnuts. Bake for 35 minutes until golden brown.