Prepare the Crust (or Use Store-Bought):

• Homemade Crust: In a large bowl, whisk together the flour and salt. Add the cold, cubed butter and use a pastry blender or your fingertips to cut the butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
• Gradually add ice water, one tablespoon at a time, mixing gently after each addition, until the dough just comes together. Be careful not to overwork the dough.
• Form the dough into a disc, wrap it in plastic wrap, and refrigerate for at least 30 minutes. This chilling time is crucial for a flaky crust as it allows the gluten to relax and the butter to solidify.
• Store-Bought Crust (Quick Option): If using a store-bought crust, simply thaw it according to package directions and proceed to step 4.

2. Caramelize the Onions:

• Heat olive oil in a large skillet over medium-low heat. Add the sliced onions and a pinch of salt.
• Cook the onions slowly, stirring occasionally, for about 20-25 minutes, or until they are deeply golden brown and caramelized. The key to caramelized onions is low and slow cooking, allowing the natural sugars in the onions to develop and deepen in flavor. Be patient and don’t rush this step.
• Once caramelized, remove the onions from the heat and set aside.

3. Prepare the Tomatoes:

• If using large tomatoes, core them and slice them into ½ inch thick rounds. If using cherry tomatoes, halve them.
• Gently pat the tomatoes dry with paper towels to remove excess moisture. This will prevent the quiche from becoming soggy.

4. Preheat Oven and Prepare Quiche Dish:

• Preheat your oven to 375°F (190°C).
• If using homemade crust, remove the dough from the refrigerator. On a lightly floured surface, roll out the dough into a 12-inch circle.
• Carefully transfer the dough to a 9-inch quiche dish or pie plate. Gently press the dough into the dish, fitting it snugly against the bottom and sides. Trim any excess dough hanging over the edge and crimp the edges decoratively if desired.
• Prick the bottom of the crust all over with a fork. This prevents the crust from puffing up unevenly during baking.

5. Blind Bake the Crust (Optional but Recommended):

• For a crispier crust, blind bake it. Line the crust with parchment paper or aluminum foil and fill it with pie weights or dried beans.
• Bake for 15 minutes. Remove the parchment paper and pie weights and bake for another 5-7 minutes, or until the crust is lightly golden. This step ensures the bottom crust is fully cooked and doesn’t become soggy from the wet filling.
• If you skip blind baking, just proceed to the next step with the unbaked crust.

6. Assemble the Quiche Filling:

• In a large bowl, whisk together the eggs, heavy cream, and milk until well combined.
• Stir in the shredded Gruyere cheese, chopped fresh thyme, salt, black pepper, and red pepper flakes (if using).

7. Layer and Pour the Filling:

• Spread the caramelized onions evenly over the bottom of the prepared crust.
• Arrange the tomato slices or halved cherry tomatoes over the onions.
• Carefully pour the egg mixture over the onions and tomatoes, ensuring it is evenly distributed.

8. Bake the Quiche:

• Bake in the preheated oven for 35-45 minutes, or until the quiche is set and the top is golden brown. The center should be just set with a slight jiggle, and a knife inserted near the center should come out mostly clean.
• If the crust is browning too quickly, you can loosely tent the edges with aluminum foil.

9. Cool and Serve:

• Remove the quiche from the oven and let it cool in the dish for at least 10-15 minutes before slicing and serving. This allows the custard to set further and makes slicing easier.
• Serve warm or at room temperature.