In a medium saucepan set over medium heat, cook the rhubarb down till just softened, but still holding its shape. Stir in the sugar and heat till it dissolves in the rhubarb's juice, then stir in the strawberries.
Cook just until bubbling, then remove from heat. Once the mixture has cooled somewhat, spoon into a clean mason jar and refrigerate until you're ready to eat! It should last about two or three weeks in the fridge.