Place the rhubarb in a saucepan with the sliced stem ginger, cinnamon stick, lemon juice, and a good splash of cold water. Bring to the boil, then cover and reduce to a simmer for 5 minutes, until tender.
Stir through the ginger syrup (from the stem ginger jar), transfer to a bowl and leave to cool, then discard the cinnamon stick.
Mix the ground cinnamon with the yogurt. Layer 4 glasses with the rhubarb and the yogurt, finishing with a few pretty pink rhubarb pieces, and chill for 30 minutes.
Top each with a sprinkling of ground cinnamon and lemon zest before serving.