My top tip: while you can make these shortcakes by pulsing the butter with the 'flour mix' in a food processor and then pouring in the egg until it all comes together (think pie crust), I've found cleanup etc. even easier by just grating the butter, freezing it (while prepping the rest) and simply gently folding everything together into a dough (see post for pictures!).
Mix peaches with lemon juice, sweetener, vanilla extract and cinnamon. Set aside and allow to macerate while you make the shortcakes.
Preheat oven to 380°F/190°C. Line a baking tray with parchment paper or a baking mat.
Add egg, sour (or coconut) cream and apple cider vinegar to a medium bowl and whisk for a minute or two until fully mixed. Set aside.
Add almond flour, flaxseed meal, coconut flour, whey protein, sweetener, baking powder, xanthan gum (or more flax), and salt to a bowl and whisk until thoroughly combined.
Add in the frozen grated butter and gently rub it into the flour, being careful not melt it in (vip!). Pour in the egg and cream mixture, and fold it all together quickly until it forms into a round.
Pat the dough onto a square (or a round!) in the prepared baking tray, dusting with coconut flour if needed. Cut out six shortcakes, working the dough as little as possible (I cut out 4 in the first round, then used the scraps for the remaining two)(and as you can see, I shaped the last one into a bit of a wonky square with my hands not to overwork the dough)(or simply cut it into 6 squares with a sharp knife if no pastry cutter is around!).
Brush with melted butter and bake for 22-25 minutes until golden. Allow to cool before serving. While they're chilling, whip up heavy cream (or cold coconut cream) in a chilled bowl until soft peaks (just!) form. I like to add a couple teaspoons of vanilla extract and allulose syrup to my whipped cream, and I suggest you follow suit!
Assemble the peach shortcakes by halving the biscuits, adding a dollop of whipped cream followed by a generous helping of peaches (roughly 1/3 of a peach will do). And be sure to finish off with a drizzle of the cinnamon 'sugar' syrup!
Store (unassembled!) in an airtight container at room temperature, for 3-4 days. You can freeze the shaped biscuit dough for 1-2 months, and bake straight from the freezer as needed.
Notes
Please see post for deets, possible subs, and (the many!) methodology pictures. I found each batch to yield 6 biscuits, and nutrition facts were estimated per biscuit without the peaches (so you can adjust the amount of these to fit your macros). But for reference, note that 1/3rd of a peach (my serving suggestion) comes to about 3-4g net carbs (= roughly 7g net per peach shortcake!).