Preheat the oven to 400 degrees Fahrenheit. Line a baking sheet with parchment paper. This prevents sticking and makes cleanup easy.
In a large mixing bowl, combine the 2 1/2 cups of white bread flour, 1/2 teaspoon salt, 1 teaspoon cream of tartar, 1 teaspoon baking soda, 1/2 cup sugar, and 1 teaspoon apple pie spice. Whisk these together until they're well combined. Making sure these dry ingredients are evenly mixed is important so the baking soda and cream of tartar are distributed throughout.
Add the 1/3 cup of cold, cut-up butter to the dry ingredients. Using a pastry cutter, two knives, or your fingertips, work the butter into the flour mixture until it resembles coarse breadcrumbs or sand. The key is keeping the butter cold and separate. You want to see small pieces of butter, not a smooth dough. This takes a few minutes of work. Don't rush it or the scones won't be as tender and flaky.
Pour the 1 1/4 cups of room-temperature sourdough starter into the flour and butter mixture. Using your hands, mix the starter into the dry ingredients until everything clings together. The dough will be somewhat sticky. This is normal and correct. Don't overmix. Mix just until everything comes together as a rough dough.
Turn the sticky dough out onto a lightly floured surface. Divide the dough into 4 equal pieces. Don't knead or work the dough too much. Scone dough should be handled gently.
Take one piece of dough and press it into a round shape roughly 1/2 inch thick. Repeat with the remaining 3 pieces. The thickness is important. Too thin and they'll dry out. Too thick and they won't bake through properly. Roughly 1/2 inch is the sweet spot.
Using a sharp knife, cut each round into 4 wedges, like cutting a pizza. This creates 16 scones total from the 4 rounds. Make clean cuts so the edges are defined.
Place the cut wedges on the parchment paper-lined baking sheet, about 1 inch apart. Spacing is important because the scones will expand slightly during baking and need room to do so. If they're too close together, they'll bake into each other and create one large mass instead of individual scones.
Brush the top of each scone lightly with the 2 tablespoons of heavy cream. Sprinkle coarse sugar generously over the tops. The cream helps the sugar adhere and also adds richness that creates a nice golden exterior. This step is what makes these look bakery-quality.
Bake for approximately 15 minutes, or until they just start to turn golden. Don't overbake. Golden just barely is the goal. Overbaked scones become dry and hard. Start checking around 12 minutes to see how quickly your oven bakes them. The scones should be cooked through but still tender and moist inside.
Remove from the oven and let cool on the baking sheet for a few minutes. These are best served warm or at room temperature. Serve with jam, clotted cream, butter, or just on their own.
