Position a rack in the upper third of the oven and preheat to 425ÂșLine a baking sheet with parchment paper.
In a large bowl, whisk together the millet, oat and tapioca flours with the cornstarch, sugar, chia seed, baking powder, and salt. Add the butter and lemon zest, and rub with your fingers until the mixture looks like gravel with some pea-sized butter bits remaining. Add the ricotta and cream, and stir / knead with your hands until the dough comes together in a rough ball. The dough should feel fairly firm, but evenly moistened.
Working quickly to keep the dough cold, turn the dough out onto a surface dusted lightly with oat flour and form it into a disk. Cut the disk into six equal pieces. Shape each piece into a ball, place on the sheet pan spaced well apart, and flatten slightly. Brush the tops of each biscuit with cream and sprinkle with a bit of coarse sugar. (If the biscuits have become soft, you can chill or freeze them until they have firmed up a bit; this will keep them from spreading too much.)
Bake the biscuits until golden on top, 14-18 minutes. Let cool at least 30 minutes; they are still cooking from residual heat. The biscuits are best the day of baking, but they will keep at room temperature for a day or two. Toast before serving.