Pulse the flour, butter, sugar, and kosher salt in a food processor until a crumbly meal forms, about 30 seconds. Add the sour cream and process until evenly mixed, 15 seconds more. Add the water 1 tablespoon at a time and pulse until the dough just comes together, 30 seconds. Wrap the dough in plastic wrap and refrigerate for at least 1 hour.
Heat oven to 400° F, with the racks positioned in the middle third. Divide the dough into 4 pieces. Cut four 12-by-17-inch sheets of parchment paper. Working with one piece of dough at a time, place the dough on a parchment sheet and, with a floured rolling pin, roll the dough until it’s paper thin (about 10 inches in diameter). Transfer to a rimmed baking sheet.
Brush the dough with the egg white and arrange 15 of the sage leaves on top, brushing them with a bit more egg white to stick. Sprinkle the dough with ½ teaspoon of the sea salt. Repeat with the remaining parchment, dough, egg white, sage, and sea salt.
Bake, 2 at a time, until the crackers are nicely browned and bubbled, 8 to 10 minutes. Transfer to a rack and let cool completely, 30 minutes. Break into pieces. Store in an airtight container at room temperature for up to 5 days.
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