TRIPLE-THREAT ONION GALETTE

Steps:

• Dough:
• Whisk sugar, salt, and 1½ cups flour in a medium bowl to combine. Add butter and toss to combine. 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).1 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.
• Turn out dough onto a lightly floured surface. Working quickly and using a rolling pin and your hands, press dough together to form a mass. 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, taking your time to make it neat as this will be your final fold. Using rolling pin, gently flatten dough, rotating as needed, to make a 6"-diameter disk.
• Dust surface with more flour and roll disk into a 14"-diameter circle or oval about ⅛" thick. Fold dough in half, then in half again to make a rough triangle (this will make it easier to move). Slide onto a parchment-lined baking sheet and unfold. Chill while you make the filling.
• Do Ahead: Dough can be made 3 days ahead. Wrap 6" disk tightly and chill until ready to roll out.
• Assembly:
• Place a rack in middle of oven; preheat to 375°F. Heat 3 Tbsp. 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.
• Melt remaining 1 Tbsp. butter in reserved skillet (no need to wipe 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.
• Remove dough from refrigerator and, using a small offset spatula or a spoon, spread mustard in a thin layer over dough, leaving a 1½"-2" border around the edges. Scatter onion mixture over mustard and sprinkle with Parmesan; season with pepper. Cutting from edge of dough to the start of the mustard and filling, make slits in border of galette at 2"-3" intervals. Fold flaps up and over filling, slightly overlapping. Scatter scallions over. Bake galette until crust is deep golden brown and scallions are lightly charred, 45-50 minutes. Using parchment paper, slide galette onto a cutting board and cut into wedges.
• Do Ahead: Onion can be cooked 3 days ahead; transfer to an airtight container, then cover and chill. Galette can be assembled 1 hour ahead; chill.