Preheat the oven to 425ºF (220ºC) and line a baking sheet with parchment paper.
Whisk together the cream, sugar, and egg in a large bowl.
If you haven't chopped your rhubarb and ginger, do so now. Cut each piece of rhubarb into three or four segments (depending on how long your stalks are). Slice each segment lengthwise into three strips (or four if your stalks are huge). Gather the strips of rhubarb together and carefully chop them into small bits, about ⅓ inch (roughly 1 cm) in size. Peel and mince the fresh ginger.
Stir the chopped rhubarb and minced ginger into the egg and cream mixture. Set aside.
In another bowl, whisk together the flour, baking powder, and salt. Add the butter to the flour and cut it in using a pastry blender or two butter knives (for information about butter temperature see the notes section below). The mixture should look coarse and crumbly.
Make a well in the middle of the flour mixture. Pour the rhubarb and cream into the well. Using a rubber spatula or wooden spoon, fold everything together until most of the flour is moistened. Switch to using your hands and knead the dough inside the bowl about 10 times.
Turn the dough out onto a lightly floured work surface then sprinkle the top of the dough with more flour. Use your fingertips to gently press the dough into a round disc, and use your hands to shape and smooth the edges. The disc should be approximately 7-½ inches (19-cm) in diameter, and about 1-inch (2-½ cm) tall.
Brush the top and sides of the disc with whipping cream then sprinkle the top generously with coarse sugar if using. Using a sharp knife, slice the disc into eight wedges. Use an offset spatula or butter knife to lift each wedge from the work surface onto the prepared baking sheet. Leave as much space between the scones as you can.
Place the sheet in the oven and bake the scones for 15 to 17 minutes, rotating the pan halfway through baking. The scones are done when the outsides are light gold in colour. Cool the scones on a wire rack, on the baking sheet, for 10 minutes before removing them from the sheet and cooling them directly on the rack.