To make the crust: In the bowl of a food processor, pulse the flour and salt to combine. Scatter the cubed butter over the flour and pulse until the butter is in pea-size pieces., Drizzle in half of the cold water and pulse a few times. Add the remaining water and pulse a few more times, just until the dough begins to come together in a ball. Turn the dough out onto a sheet of plastic wrap. It may still be a bit crumbly; that's OK. Use the plastic wrap to help gather the dough, then tightly wrap and flatten it into a disk. Note: You can also make this dough by hand, or using a stand or hand mixer. , Refrigerate the dough until cold, at least 30 minutes or up to overnight., To make the filling: Preheat the oven to 375°F. Toss the sliced red onion with the olive oil and 1/2 teaspoon kosher salt. Spread on a small rimmed baking sheet and bake, stirring occasionally, until the onion is soft and slightly frizzled, 25 to 30 minutes., Heat a few inches of water in a large saucepan and add the broccoli. Cook just until tender, then drain and rinse with cold water. Set aside., To assemble: Remove the crust from the refrigerator. On a floured work surface, roll the crust into a 12" circle. Carefully transfer to a high-sided 9" tart pan with a removable bottom; crimp the edges., Transfer the crust to the refrigerator and chill until cold, about 30 minutes, while you preheat the oven to 375°F., Line the chilled shell with parchment and fill with pie weights (or uncooked rice or dried beans). Place on a baking sheet and bake for 20 minutes. Carefully remove the weights and parchment and continue baking for 10 to 15 minutes more, until the crust is golden brown. Remove from the oven and cool., In a large bowl, whisk together the crème fraîche, heavy cream, flour, eggs, salt, thyme, and black pepper., Line the cooled crust with the broccoli and half of the onions. Pour the crème fraîche mixture over, then scatter the blue cheese and remaining onions over the top., Transfer the tart (still on the baking sheet) to the oven and bake for 10 minutes. Reduce the oven temperature to 325°F and continue baking until the custard is set and the top is brown, about 40 minutes longer., Remove from the oven and let cool until just warm, then cut into wedges and serve., Store any leftovers, covered, in the refrigerator for up to three days.