PREP: Preheat oven to 400 degrees with a rack in the upper third. Line a baking sheet with parchment paper.
MIX DRY INGREDIENTS/CUT IN BUTTER: In the bowl of a food processor fitted with a steel blade (or in a large bowl using a pastry cutter or your fingers), measure in the flour, sugar, baking powder, and salt. Pulse or stir a few times to mix. Add the pieces of butter and pulse (or cut in/rub in) carefully 6-8 times or until you have pea-sized and smaller pieces of butter.
TURN OUT FLOUR MIXTURE INTO A LARGE BOWL and stir in the peaches. Whisk together the sour cream or yogurt and milk and pour that into the bowl. Using a table fork, stir until all the dry bits are incorporated, but the dough is still shaggy. Knead a few times until the dough barely holds together, then turn it out onto a lightly floured counter or board. The dough shouldn’t be uniform, smooth, or neat. Do not overwork dough.
PAT DOUGH with floured hands into a 1-inch-thick rectangle approximately 6 ½” x 12 ½”. Brush with the egg mixture and sprinkle with a little granulated sugar. Using a bench scraper or a knife, cut into first 2, then 4, then 8 (or then 16 for small scones) rectangles. (See Cook's Notes on info for even smaller scones.) for Place on prepared sheet pan as far apart as possible.
BAKE scones until golden, 18-22 minutes. Remove to a rack and cool for a minute or two before serving warm. Scones will keep for a day or two, well-covered but are best when fresh. To reheat, split in half lengthwise, spread lightly with butter, and grill until toasty and hot in a stove top grill pan or skillet. Freeze extras in a Ziplock freezer bag for 2 months.