For the crust: Follow the recipe’s instructions to make the shortcrust pastry. Transfer the pastry out of the food processor bowl and onto a lightly floured, clean working surface. Gather into a thick, flat round. Lightly sprinkle with flour, then roll out to a 14-in (35 cm) circle.
Let the shortcrust pastry rest for 15 to 30 minutes, while you prepare the rest of the components. (If your kitchen is very warm, transfer the rolled-out pastry to a baking sheet and refrigerate for the same amount of time. You can also make the crust in advance and refrigerate it for up to a half-day.)
For the crostata: Line a baking sheet with two layers of paper towels. Cut the heirloom tomatoes into ½-in (1 ¼ cm) thick slices and set flat, side-by-side, over the paper towel-lined baking sheet. Sprinkle with fleur de sel or sea salt, flip over, then sprinkle the second side with salt, too. Let rest for 15 minutes.
For the homemade pesto: To the bowl of a food processor, add all the ingredients, except the water. Process until smooth, scraping down the sides as needed. This pesto should be a bit thicker than one you would use for pasta, but it should still be easily spreadable. If the pesto seems very thick, or if you have trouble reaching a smooth consistency, add water 1 or 2 tsp (5 to 10 ml) at a time and process until you reach the right consistency.
To bake the heirloom tomato crostata: Preheat the oven to 425°F (210°C). If you refrigerated the shortcrust pastry, bring it back to room temperature 10 minutes before assembling the crostata. Line a baking sheet with parchment paper, then place the rolled out shortcrust pastry over this prepared sheet. (If you’re using a rectangular baking sheet, the pastry will hang over the sides at first, but the sheet will be just the right size once the galette is assembled and ready to bake.)
Spread the bottom of the crust with a thin layer of pesto, leaving a 2-inch (5 cm) border all around. (Refrigerate or freeze the remaining pesto for another use.) Dot the pesto with small chunks of creamy cheese. Pat the heirloom tomato slices dry, then add to the crostata, fanning them in circles. Bring the edges of the dough up and over the filling, creasing it and gently pressing it down onto the filling as you go. Brush the dough with milk and sprinkle with fleur de sel, or flaky sea salt.
Bake for 35 minutes, or until the crust is golden brown. Set the baking over a rack and let the heirloom tomato crostata cool completely, at least 30 minutes.
SERVING: Garnish with additional chunks of creamy cheese, some lemon zest, and fresh basil leaves. Use a serrated knife to slice and serve with a green salad.
STORAGE: Because of the high water content of tomatoes, this heirloom tomato crostata does not keep well. It's best enjoyed on the day it is baked.
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