In a large bowl, gently toss together the tomatoes and kosher salt. Let sit for at least 10 minutes and up to 30 minutes. Set up a kitchen towel on a work surface. Leaving any excess liquid in the bowl, transfer the tomatoes to the towel. Pat the tomatoes dry.

While the tomatoes are sitting, in a small bowl, stir together the fennel, cumin seeds, chile flakes, and coriander. In another small bowl, stir together the mayonnaise, garlic, and a pinch of kosher salt.

Preheat the oven to 425°F (220°C) with a rack positioned in the center. Line a sheet pan with parchment paper.

Roll the dough into a round and set it on the lined sheet pan using the Basic Method.

Spoon the garlicky mayonnaise into the crust, leaving a 2-inch (5 cm) border. Layer the tomatoes over the mayonnaise (start with larger ones, then tile in smaller ones in open spaces, overlapping as needed). Sprinkle the tomatoes with the spice mix. Fold the edges of the crust over the filling toward the center, overlapping and pleating as desired. Freeze the galette on the sheet pan for 10 minutes.

Remove the galette from the freezer and brush the egg wash over the exposed crust. Drizzle the oil over the filling.

Bake until the crust is starting to turn golden, 12 to 15 minutes. Reduce the oven temperature to 375°F (190°C) and continue to bake until the filling is starting to brown and the crust is deeply golden brown, another 40 to 50 minutes, rotating the sheet pan front to back halfway through.

Remove the galette from the oven and sprinkle with some flaky sea salt. Cool, uncovered, for 20 minutes or up to 2 hours.

Slice and serve. Leftovers can be stored in an airtight container in the refrigerator for up to 3 days. Reheat on a sheet pan in a 350°F (180°C) oven until warmed through, about 10 minutes.