Preheat the oven to 350F.

Defrost frozen rhubarb in a colander set over a bowl, and discard any melt-water that accumulates in the bowl.

Toss fresh or thawed rhubarb and cranberries, if using, with sugar and spread out in a 9 x 13 baking dish.

Roast for 40 to 60 minutes, stirring every 20 minutes, until all the liquid has evaporated and rhubarb is just beginning to caramelize at the edges.

Remove from the oven and cool to room temperature. Store in the fridge for up to two weeks and in the freezer for up to six months.