01 - Preheat the oven to 375°F.
02 - Combine flour and salt in a bowl. Cut in cold butter until mixture resembles coarse crumbs. Gradually add ice water, stirring until dough comes together. Shape into a disk, wrap in plastic, and chill for 20 minutes.
03 - Roll out the dough on a floured surface to fit a 9-inch tart pan. Press dough into the pan and trim excess edges. Prick base with a fork. Line with parchment paper and fill with pie weights or dried beans.
04 - Bake the crust with weights for 15 minutes. Remove parchment and weights, then bake for an additional 5 minutes. Set aside.
05 - Heat olive oil in a skillet over medium heat. Sauté onion for 3 minutes. Add zucchini, bell pepper, and garlic; cook for 5 minutes until softened. Stir in cherry tomatoes and cook 2 more minutes. Remove from heat.
06 - Whisk together eggs, whole milk, heavy cream, salt, black pepper, and dried thyme until combined.
07 - Spread sautéed vegetables evenly over the baked crust. Sprinkle shredded Gruyère cheese on top. Pour the egg mixture over all.
08 - Return tart to the oven and bake for 25 to 30 minutes, until the custard is set and the top is golden brown. Allow to cool slightly before slicing.