Place a rack in middle of oven; preheat to 425°. Whisk salt, baking soda, baking powder, sugar, pepper, and 4 cups flour in a large bowl. Add butter and, using your fingers, work into dry ingredients, smashing and flattening and tossing to coat with flour, until you have some flat quarter-size pieces and some smaller pea-size pieces. Add scallions, rosemary, sage,and fennel seeds and toss to evenly distribute. Add 1½ cups buttermilk and stir mixture with a fork just until a shaggy dough forms. Turn out dough onto a lightly floured surface and knead once or twice to incorporate any loose pieces. Pat dough into a 10x8" rectangle. Using a large knife, cut in half crosswise, then stack 1 piece on top of the other. Turn rectangle so a long side is closest to you and cut dough in half crosswise again. Stack 1 piece on top of the other. (This process will build layers into your biscuits.) Turn dough and pat again into a 10x8" rectangle. Cut into 12 even pieces (you should have a grid of 4 on the long sides and 3 on the short sides). Place biscuits on a parchment-lined baking sheet, spacing evenly, and freeze 10 minutes. Brush top of each biscuit lightly with buttermilk and bake, rotating baking sheet halfway through, until golden brown, 22–26 minutes.