In the bowl of a food processor, mix flour through pepper, pulsing to combine. Wait a minute for powder to settle, then add butter, pulsing until there's an even texture throughout. Transfer to a large bowl and add onions; toss to combine. With a large spoon, fold in ¾ cup sour cream until dough starts to form large chunks, adding more sour cream if needed. Press together into a ball.
On a lightly floured surface, press into a sheet; fold 1/3 of the sheet into the center, then overlap the other third. Press into a sheet again, then repeat folds. Form and press into an evenly leveled 10- to 12-inch square. Press a bit of flaky salt into top of dough, if desired. Square off edges with a bench scraper or side of knife, then crosscut into thirds to make 9 biscuits.
Preheat oven to 425 F. Space biscuits 2 inches apart on a baking sheet and bake 18 to 20 minutes, until golden brown and firm when tapped. Transfer to a cooling rack. Once fully cooled, store in an airtight container.
Per serving: 269 calories, 15 grams fat (9 grams saturated fat), 30 grams carbohydrates, 39 milligrams cholesterol, 408 milligrams sodium, 1 gram fiber, 4.5 grams protein.
