Preheat the oven to 350 degrees F. Place a quarter-sheet pan or pie plate on a rimmed baking sheet lined with foil or parchment paper.
For the pie crust: In a food processor, pulse the flour, salt and butter together until the butter is the size of peas. Add the egg yolks and pulse until well combined. While pulsing, slowly add the ice water until the dough just comes together. Tip out onto a board and wrap in plastic wrap. Chill in the fridge for 30 minutes.
When the dough is rested, roll out into a rectangle and place into the prepared baking sheet. Line with parchment paper and fill with pie weights. Blind bake for 20 minutes. Remove the weights and parchment and bake for another 10 minutes until golden. Set aside.
Turn the oven temperature up to 375 degrees F.
In a medium skillet over medium heat, drizzle in the olive oil. Add the garlic, thyme sprigs and onion and saute until translucent. Season to taste with salt and pepper. Set aside to cool; discard the thyme.
For the vegetables: Lay the zucchini, yellow squash and butternut squash on a parchment-lined half-sheet pan. Drizzle with olive oil and season with salt and pepper. Bake for about 5 minutes in the oven, just enough to partially cooked the vegetables.
For the custard: In a medium bowl, whisk together the eggs and half-and-half. Add the lemon zest, chopped thyme, and salt and pepper. Set aside.
Cover the base of the pie crust with the cooked onions and sprinkle over half of the cheese. Arrange the vegetables on top, and then pour over the custard mixture carefully. Sprinkle over the rest of the cheese.
Bake for about 30 minutes, or until the center is set. Cool for at least 20 minutes before serving.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.