Preheat the oven to 375 F. Saute the onion and zucchini in 2 tablespoons olive oil until tender and slightly caramelized. Set aside in a bowl to cool slightly.
Spray a 9- or 10-inch pie pan with nonstick cooking spray, then drizzle the remaining tablespoons olive oil to coat the bottom of the pan. Spread the thinly sliced potatoes over the bottom of the pan in a circular pattern, each piece overlapping slightly. Set aside.
In a medium size bowl, whisk the 3 eggs, cream, milk, Parmesan, Manchego and Boursin to combine. Season with 1/2 teaspoon salt and 1/2 teaspoon black pepper.
Add the sauteed vegetables to the egg and milk mixture, stir to combine. Pour the mixture over the potatoes in the pan.
Cut a large 11-inch circle out of the puff pastry. Place the pastry on top of the filling in the pan. Gently fold the edges under and crimp. Brush the beaten egg on top and sprinkle generously with sesame seeds.
Bake for 45-50 minutes, until the top is a deep golden brown.