Step 1: Prepare the Crust (If Making Homemade)

1. Combine Dry Ingredients: In a large bowl, whisk together the flour and salt. This ensures the salt is evenly distributed throughout the dough.
2. Incorporate Cold Butter: Add the cold, cubed butter to the flour mixture. Using a pastry blender, your fingertips, or a food processor, cut the butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. The goal is to keep the butter cold and in small pieces, which will create flaky layers in the crust. If using your fingertips, work quickly to prevent the butter from melting. If using a food processor, pulse in short bursts to avoid over-processing.
3. Add Ice Water and Vinegar (Optional): Gradually add the ice water, one tablespoon at a time, mixing gently after each addition. If using vinegar or lemon juice, add it along with the first tablespoon of ice water. Mix until the dough just comes together and forms a shaggy ball. Be careful not to overmix, as this will develop the gluten and result in a tough crust.
4. Form and Chill the Dough: Turn the dough out onto a lightly floured surface and gently gather it into a disc. Wrap the disc tightly in plastic wrap and refrigerate for at least 30 minutes, or up to 2 hours. Chilling allows the gluten to relax, making the dough easier to roll out and preventing it from shrinking during baking.

Step 2: Prepare the Asparagus and Filling

1. Prepare the Asparagus: Wash the asparagus thoroughly and snap off the tough woody ends. You can snap them off naturally, or trim about the bottom inch of each spear. Cut the asparagus spears into 1-2 inch pieces. Smaller pieces will be easier to arrange in the tart and will cook more evenly.
2. (Optional) Sauté the Asparagus: For a slightly more tender asparagus and to enhance its flavor, you can sauté it briefly. Heat olive oil in a skillet over medium heat. Add the minced garlic and cook for about 30 seconds until fragrant, being careful not to burn it. Add the asparagus and sauté for 2-3 minutes, until it turns bright green and is slightly tender-crisp. Season with salt and pepper. This step is optional; you can also use raw asparagus in the tart, which will retain a bit more of its crisp texture.
3. Whisk the Eggs and Cream: In a medium bowl, whisk together the eggs, heavy cream (or half-and-half), shredded cheese, thyme, salt, and pepper. Whisk until the mixture is well combined and smooth. Taste and adjust seasoning as needed.

Step 3: Assemble and Bake the Tart

1. Preheat Oven and Prepare Tart Pan: Preheat oven to 375°F (190°C). Lightly grease a 9-inch tart pan with a removable bottom. This ensures the tart releases easily after baking.
2. Roll Out the Crust (If Homemade): On a lightly floured surface, roll out the chilled dough into a 12-inch circle, about ⅛ inch thick. Gently transfer the dough to the prepared tart pan, pressing it into the bottom and up the sides. Trim any excess dough hanging over the edge. You can crimp the edges for a decorative look, or simply leave them as is.
3. Blind Bake the Crust (Optional but Recommended for a Crisp Crust): For a particularly crisp crust, especially if using a homemade crust, blind bake it before adding the filling. 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 brown. Blind baking pre-cooks the crust, preventing it from becoming soggy when the wet filling is added.
4. Arrange Asparagus in Crust: Arrange the asparagus pieces evenly over the bottom of the tart crust. If you sautéed the asparagus, distribute it evenly. If using raw asparagus, arrange it in a decorative pattern, if desired.
5. Pour Filling Over Asparagus: Pour the egg and cheese mixture evenly over the asparagus in the crust. Ensure the filling is distributed throughout the tart.
6. Bake the Tart: Bake in the preheated oven for 30-40 minutes, or until the crust is golden brown and the filling is set and lightly golden on top. A toothpick inserted into the center should come out clean, and the filling should no longer jiggle.
7. Cool and Serve: Let the tart cool in the pan for at least 10-15 minutes before removing the sides of the tart pan. This allows the filling to set further and makes it easier to slice and serve. Serve warm or at room temperature.