Place rhubarb, strawberries, water and sugar in a medium-sized pot set over medium heat. Cook, stirring often, until the juices are released and begin to cover the fruit. Pour the mixture through a fine mesh sieve to separate the fruit from the juices. Set the fruit aside.
Return the the juices to the pot and bring to a boil over medium-high heat. Allow to cook, stirring frequently, until reduced by approximately half (an easy way to check this is to use the handle of a wooden spoon. Dip it into the liquid to “stain” it. Then use that as a measuring guide for how much it’s cooking down). Skim off any foam that rises to the surface, as you go.
Add the fruit back to the pot, along with any accumulated juices. Stir in lemon juice. Bring to a simmer and cook until a spoonful of the mixture mounds on a frozen plate without spreading. Once ready, you can pack it into jars, or tupperware, and freeze. Or, follow the process below for canning.