Step 1

First, make the crust. Whisk 1½ cups (188 g) all-purpose flour, 1 Tbsp. sugar, and 1½ tsp. Diamond Crystal or 1 tsp. Morton kosher salt in a medium bowl to combine. Cut 12 Tbsp. (1½ sticks) chilled unsalted butter into ½” pieces, add to dry ingredients, and toss to coat and distribute. Using your hands and working quickly, rub and smash butter into flat irregular pieces (be careful not to overwork; you don’t want to soften the butter too much. Just make sure you smash each piece at least once. And if the butter is getting too warm, throw the bowl in the fridge or freezer and let chill for a bit before proceeding).

Step 2

Drizzle ⅓ cup cold water over and mix with a rubber spatula, smashing in butter, until dough mostly comes together—it will be dry and shaggy.

Step 3

Now you are going to fold the dough, which will give it distinct flaky layers when baked. Turn out onto a lightly floured surface. Working quickly and using a rolling pin and your hands, press dough together to form a mass. (The dough will seem like a crumbly dry mess at first but will come together as you fold and roll. Have faith and don’t give up.) Roll dough into a rough square about ½” thick. Fold in half into a rectangle, then once again to make a small square. Press down on the dough with rolling pin to make it hold together. Roll dough out once more to make a rough square about ½” thick. Repeat folding process again, taking your time to make it neat, as this will be your final fold.

Step 4

Using rolling pin, gently flatten dough, rotating as needed, to make a 6”-diameter disk. Dust surface with more flour (go heavy; you don’t want the dough to stick) and roll disk into a 14”-diameter circle or oval about ⅛” thick. Brush off any excess flour with a pastry brush. Fold dough in half, then in half again to make a rough triangle (this just makes it easier to move). Transfer to a parchment-lined baking sheet and unfold (it’s okay if dough is a bit larger than the baking sheet). Chill while you make the filling.

Step 5

Place a rack in middle of oven; preheat to 375°. Peel and thinly slice 1 large onion, then 8 garlic cloves.

Step 6

Heat 3 Tbsp. unsalted butter in a medium skillet over medium until foaming, about 1 minute. Reduce heat to medium-low; add onion, garlic, and 1 tsp. Diamond Crystal or ½ tsp. Morton salt, and stir to combine. Cover and cook, stirring occasionally, until onion is tender and just beginning to brown, 6–8 minutes. Uncover and continue to cook, stirring occasionally and adding a splash of water if onion begins to stick, until golden brown in spots, about 5 minutes more. Transfer onion mixture to a plate; reserve skillet.

Step 7

While the onion mixture is cooking, finely grate 2 oz. Parmesan (you should have about 2 cups). Trim 12 scallions, then thinly slice on a steep diagonal. This will yield striking oblique disks, but if you’re nervous about your knife skills, simply slice crosswise.

Step 8

Melt remaining 1 Tbsp. unsalted butter in reserved skillet (no need to wipe it out) over medium heat. Remove from heat and add scallions and remaining ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt. Toss scallions until coated in butter.

Step 9

Now you are ready to assemble your galette! Remove dough from refrigerator and, using a small offset spatula or spoon, spread 2 Tbsp. Dijon mustard in a thin layer over dough, leaving a 1½”–2” border around the edges. Scatter onion mixture over mustard and sprinkle with cheese; season with freshly ground black pepper.

Step 10

Cutting from edge of dough to the start of the mustard and filling, make slits in border of galette at 2”–3” intervals (the slits are optional but make for a more beautiful presentation). Fold flaps up and over filling, slightly overlapping. Scatter scallions over.

Step 11

Bake galette until crust is deep golden brown and scallions are lightly charred, 45–50 minutes. (Your crust isn’t properly baked until it’s the color of a well-worn penny. If your timer dings but the dough is pale, keep going.) Using parchment paper, slide galette onto a cutting board and cut into wedges.

Step 12

Do ahead: Dough can be made 3 days ahead; wrap 6”-diameter disk tightly and chill until ready to roll out. Onion mixture can be cooked 3 days ahead; transfer to an airtight container, then cover and chill. Galette can be assembled 1 hour ahead; chill.