Preheat the oven to 400°F.
Cut the rhubarb into 2-inch lengths and put in a baking pan with a good pinch of cinnamon, if using, the orange juice, and 2 tablespoons of the sugar. Toss well and roast in the oven for 10 to 15 minutes, or until tender and just beginning to caramelize around the edges. Let cool completely and then drain in a sieve.
Turn the oven temperature down to 350°F. Lightly butter a 10-inch round baking dish or an 8-by-10-inch rectangular one. Make the batter by sifting the salt and flour into a bowl and stirring in the sugar. Make a well in the center and add the beaten eggs. Gradually stir in the flour from the sides, mixing well, then beat in the milk, a little at a time, mixing well. Stir in orange zest and a pinch of cinnamon, if you like.
Arrange the drained rhubarb in the buttered dish and pour the batter over. Bake for 35 to 40 minutes, until golden and puffed up. Clafoutis is best when eaten warm, but it's not bad when cold, either. When you are just about to serve, mix together about ½ teaspoon confectioners’ sugar and ½ teaspoon ground cinnamon, if you like, then sift a light dusting over the top of the clafoutis. Serve with or without cream.