In a large bowl whisk together the oat flour, millet flour, tapioca flour, corn starch, sugar, baking powder, salt, and lemon zest.
Add the butter pieces. Use your finger tips to work the butter into the dry ingredients, until the butter is the size of small peas.
Chill in the refrigerator until cold (20-30 minutes).
Remove the flour mixture from the refrigerator and add the ricotta and 3 tbsp of cream.
Stir until the dough comes together in a rough ball.
Stack a rimmed baking sheet on top of a second rimmed baking sheet, and line with parchment paper.
Working quickly to keep the dough cold, put the dough on a surface dusted lightly with oat flour, and form it into a disc. Cut the disc into six equal pieces, shaping each into a ball. Place on the sheet pan, and flatten slightly. Chill the shortcakes in the refrigerator 30-60 minutes until firm.
Position a rack in the upper third of the oven, and preheat to 425° F.
Brush the top of each shortcake with the remaining two teaspoons of cream. Sprinkle with the coarse sugar.
Bake on the upper rack of the oven until golden on top (15-20 minutes).
On a wire rack, let cool completely.
For the mascarpone cream
Add all the ingredients to the bowl of a stand mixer, fitted with the whisk attachment, and whisk on medium speed until the cream holds soft peaks. Chill in the refrigerator until ready to use.
To Assemble
Toss the sliced peaches with the honey and tarragon, and let sit for 10 minutes.
Use a sharp serrated knife to slice the shortcakes in half horizontally.
Place the bottom half of the shortcake on a plate, add a dollop of mascarpone cream, and top with a large spoonful of honeyed peaches. Add the shortcake top, drizzle with more honey, and garnish with a few fresh tarragon sprigs.