Step 1

Whisk flour, sugar, and salt in a large bowl to combine. Whisk together yogurt and oil in a small bowl (it won’t be perfectly smooth). Make a well in center of dry ingredients and scrape in yogurt mixture. Using a fork, gradually incorporate dry ingredients into yogurt mixture, then use your hands to work together until a shaggy dough forms (you can also turn out onto a surface if that’s easier). If the dough doesn’t come together in 1 piece, knead in 1 Tbsp. ice water. Shape dough into a 1”-thick disk. Wrap in plastic and chill at least 2 hours and up to 12 hours.

Step 2

Heat oil in a large skillet over medium. Add leek and cook, stirring, until softened and wilted (don’t let it take on any color), 6–8 minutes. Add garlic and red pepper flakes and cook, stirring, just until garlic begins to soften, about 1 minute. Add Swiss chard by the handful, letting each addition wilt slightly before adding more; cook, stirring often, just until chard is tender, about 4 minutes. Remove from heat; season with salt and pepper. Transfer Swiss chard mixture to a large bowl and let cool 20 minutes. Mix Parmesan into filling.

Step 3

Preheat oven to 400°. Unwrap dough and roll out on a lightly floured sheet of parchment paper to a 12” round about ⅛” thick. Slide dough and parchment onto a baking sheet. Scatter filling over dough, leaving a 1½” border. Bring edges of dough up and over filling, overlapping as needed; brush dough with egg. Chill in freezer 10 minutes.

Step 4

Bake galette, rotating front to back halfway through, until crust is golden brown and cooked through, 40–50 minutes. Let cool slightly on baking sheet.

Step 5

To serve, dollop filling with yogurt if desired and top with herbs and lemon zest.