Step 1: Prepare the Crust (If making homemade)

1. Combine Dry Ingredients: In a large bowl, whisk together the flour and salt.
2. Incorporate Butter: Add the cold, cubed butter to the flour mixture. Using your fingertips or a pastry blender, cut the butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. The key here is to keep the butter cold, which will create flaky layers in the crust.
3. Add Ice Water: Gradually add ice water, one tablespoon at a time, mixing gently after each addition until the dough just comes together. Be careful not to overmix; you want to avoid developing too much gluten, which can make the crust tough.
4. Form a Disc and Chill: Form the dough into a disc, wrap it in plastic wrap, and refrigerate for at least 30 minutes (or up to 2 hours). This chilling period allows the gluten to relax and the butter to firm up, making the dough easier to roll out.
5. Roll Out the Dough: On a lightly floured surface, roll out the chilled dough into a circle about 12 inches in diameter and about ⅛ inch thick.
6. Line the Pie Dish: Gently transfer the rolled-out dough to a 9-inch pie dish. Press the dough into the bottom and up the sides of the dish. Trim any excess dough and crimp the edges decoratively, if desired.
7. Pre-bake the Crust (Blind Baking – Optional but Recommended for a Crisper Crust): Preheat oven to 375°F (190°C). Line the crust with parchment paper and fill with pie weights or dried beans. Bake for 15 minutes. Remove the parchment paper and weights and bake for another 5-7 minutes until the crust is lightly golden. Blind baking helps prevent the bottom crust from becoming soggy when filled with the moist quiche filling. If you prefer a softer crust, you can skip blind baking.

Step 2: Prepare the Filling

1. Sauté Aromatics: Heat olive oil or butter in a skillet over medium heat. Add the chopped onion or shallot and sauté until softened and translucent, about 5 minutes. Add the minced garlic (if using) and cook for another minute until fragrant. Be careful not to burn the garlic.
2. Combine Wet Ingredients: In a large bowl, whisk together the eggs, heavy cream (or half-and-half), and milk until well combined.
3. Add Cheese and Seasonings: Stir in the grated cheese, dried herbs, salt, pepper, and red pepper flakes (if using) into the egg mixture. Mix until the cheese is evenly distributed.
4. Prepare Tomatoes: If using sliced tomatoes, pat them dry with paper towels to remove excess moisture. If using diced tomatoes, drain any excess liquid.

Step 3: Assemble and Bake the Quiche

1. Preheat Oven: Preheat oven to 375°F (190°C).
2. Arrange Tomatoes and Sautéed Vegetables in Crust: If using pre-baked crust, evenly distribute the sautéed onion/garlic mixture in the bottom of the crust. Arrange the tomato slices or diced tomatoes over the vegetables. If you skipped blind baking, simply place the sautéed vegetables and tomatoes directly into the unbaked crust.
3. Pour in Egg Mixture: Gently pour the egg and cheese mixture over the vegetables in the crust, ensuring it’s evenly distributed.
4. Bake: Bake in the preheated oven for 35-45 minutes, or until the quiche is set and the crust is golden brown. The center should be just slightly jiggly but not liquidy. To check for doneness, gently insert a knife into the center of the quiche; it should come out mostly clean.
5. Cool Slightly and Garnish: Let the quiche cool in the pie dish for at least 10-15 minutes before slicing and serving. This allows the filling to set further and makes it easier to cut. Garnish with fresh basil or parsley, if desired.