Preheat the oven to 400F.
In a medium bowl, stir together the flour, sugar, baking powder, baking soda and salt.
In a small bowl or measuring cup, stir together the butter, sour cream and egg. Add to the dry ingredients along with the rhubarb and stir just until combined.
Drop in large spoonfuls (about 1/3 cup each) onto a parchment-lined baking sheet and sprinkle with coarse sugar. Bake for 15-20 minutes, or until golden. Makes 6-8 scones.