Sour Cream and Onion Biscuits

2½ tsp. Diamond Crystal or 1½ tsp. Morton kosher salt
1 tsp. freshly ground black pepper
1¾ tsp. baking powder
½ tsp. baking soda
1¼ tsp. sugar
2½ cups (313 g) all-purpose flour, plus more for surface
10 Tbsp. chilled unsalted butter, cut into (½") pieces, plus 2 Tbsp. melted
8 scallions, thinly sliced crosswise (not on a diagonal)
1¼ cups sour cream, plus more for serving
Flaky sea salt

Place a rack in middle of oven; preheat to 425°F. Whisk salt, pepper, baking powder, baking soda, sugar, and 2½ cups flour in a large bowl to combine. Add chilled butter and toss to coat. Using your hands, work butter into dry ingredients until there are lots of flat pieces and pea-size bits. Add scallions and toss to evenly distribute.
Create a well in the center of mixture and add 1¼ cups sour cream into the center. Using a fork and working in circles, mix until large shaggy clumps form. Turn dough out onto a lightly floured surface and knead once or twice until it comes together (flour your hands if needed). If there are any loose bits, add them to center of dough and knead once more to incorporate. Pat dough into an 8x4" rectangle about 1" thick.
Working from a short side, fold dough in thirds as you would a letter. It doesn’t need to overlap perfectly. Pat dough into another 8x4" rectangle, then fold dough in thirds like a letter one more time. Pat dough back into an 8x4" rectangle and cut in half lengthwise. Cut each half crosswise into 4 squares for a total of 8 biscuits.
Transfer biscuits to a parchment-lined baking sheet. Brush tops gently with melted butter; sprinkle with sea salt. Bake biscuits until golden brown, 18–22 minutes. Serve warm with sour cream or more butter for slathering.