Dough
Step 1
Whisk 1¾ cups plus 1 Tbsp. (227 g) all-purpose flour, 2 tsp. sugar, 1½ tsp. coarsely ground pepper, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt in a large bowl to combine. Add 13 Tbsp. chilled unsalted butter, cut into ½” pieces, and toss to coat. Using your fingers, smash and rub butter into dry ingredients to create small, flat pieces.
Step 2
Combine 2 tsp. apple cider vinegar and ¼ cup water in a small bowl. Drizzle half of vinegar mixture over flour mixture. Starting from bottom of bowl and working up, mix with your hands (like tossing a salad) until mixture starts to look dry again. Drizzle in remaining vinegar mixture and toss again to combine. The mixture should be very crumbly but without any dry spots. Toss a couple more times, then gently squeeze a handful in your palm; it should hold together but break apart easily. If it seems very dry, drizzle in a bit more water, toss to combine, and try squeezing again.
Step 3
Turn dough out onto a surface and pat into a rectangle about ½” thick. Using a bench scraper, fold dough in half over onto itself and press down to flatten to about ½” thick. (Dough shouldn’t be moist or sticky, but dust lightly with flour if it is.) Repeat folding and pressing 2 more times. Dough should go from shaggy to cohesive as you fold and butter will blend all the way in so surface of dough looks like pepper-flecked marble. Pat into a 9×3½” rectangle. Wrap in plastic and chill at least 2 hours.
Do Ahead: Dough can be made 5 days ahead. Keep chilled.
Filling and Assembly
Step 4
Place racks in upper and lower thirds of oven; preheat to 400°. Arrange three 3″–4″-diameter heirloom tomatoes, cored, sliced ½” thick, in a single layer on a kitchen towel. Sprinkle lightly with kosher salt on both sides. Let sit until ready to use (salting will pull out excess moisture from tomatoes).
Step 5
Whisk together 8 garlic cloves, finely grated, 2 oz. finely grated Parmesan (about 1 cup), ½ cup plus 2 Tbsp. mayonnaise, and ⅓ cup finely chopped mixed tender herbs (such as chives, dill, parsley, and/or basil) in a large bowl; chill until ready to use.
Step 6
Unwrap dough and cut in half to make two 4½x3½” rectangles. Rewrap 1 half of dough and return to fridge. Let other half sit at room temperature 5 minutes to soften slightly.
Step 7
Roll out dough to an 18×6¼” rectangle about ⅛” thick, dusting with all-purpose flour and turning as needed to avoid sticking. Using a 6″-diameter cookie cutter or bowl and a small knife, cut out 3 rounds. Place on a parchment-lined baking sheet. Gently press together dough scraps into a flat mass and place on a plate; chill. Repeat process with remaining dough half, arranging rounds on another parchment-lined baking sheet. (You should have 6 dough rounds divided between 2 baking sheets.) Roll first batch of dough scraps into a 6¼” square about ⅛” thick. Cut out another round. Repeat with second batch of dough scraps. Place 1 round on each baking sheet (for 4 dough rounds on each baking sheet); discard remaining scraps.
Step 8
Divide filling among dough rounds (about 2 Tbsp. each) and spread over surface, leaving a 1½” border around edges. Working one at a time, brush border of rounds with 1 large egg, beaten to blend, then fold and pleat dough up and over itself, right up to edges of filling, gently pressing to adhere. Top each galette with a tomato slice. Chill galettes at least 15 minutes. (If baking from frozen, no need to chill.) Brush top of crust with more beaten egg (make sure each one is well coated).
Step 9
Bake galettes 15 minutes. Rotate baking sheets top to bottom and front to back and continue to bake galettes until crust is deeply golden brown, 15–20 minutes. Let cool 5 minutes on baking sheets, then transfer to wire racks and let cool at least another 15 minutes.
Step 10
Just before serving, top galettes with coarsely chopped mixed tender herbs and flaky sea salt.
Do Ahead: Galettes can be formed 2 months ahead; wrap baking sheets in plastic and freeze. Galettes can be baked 5 days ahead. Loosely cover and chill. Reheat in a 350° oven until warmed through, 10–15 minutes.