Wash the rhubarb stems and trim both ends so that you remove any leaves and the root end. Slice the stems in chunks, roughly 1 ½in (3 - 4cm) long.
Place the rhubarb chunks in a baking dish that allows you to form a single layer, as far as possible (I used an 8x8 in dish, 2x20cm). Sprinkle over the orange zest, sugar and orange juice then toss everything together and spread out into a single layer.
Cover the dish with foil then roast for approximately 15 minutes. By this time, you should have juices formed and the rhubarb should be mostly tender to a knifepoint. If not, give it a few more minutes. Remove the foil and spoon some of the syrup over the rhubarb chunks. If you are finding the rhubarb at the edges is cooking quicker, you can move some chunks around but take care as you do as the rhubarb may break up.
Return the dish to the oven, uncovered, and roast for another 5 or so minutes to help the syrup to thicken up a little. Serve warm or allow to cool and carefully transfer to a dish to store in the fridge until needed.
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Notes
If you'd like to add some additional flavors, you could add a little ground cinnamon, ginger or cloves - I'd suggest no more than around ½ teaspoon (2.5ml) for this quantity. Alternatively, you could add a little vanilla, again a similar amount. If you want to use this to make rhubarb fool, I'd suggest cutting the chunks a little smaller.