Add rhubarb, sugar, and water to a large saucepan. Bring to a boil, lower heat and cook at a simmer 5 to 7 minutes until rhubarb is breaking down and tender.
Add strawberries and cook another 3 minutes, stirring often.
Remove from heat and blend mixture with an immersion blender in the pan until smooth (alternately, carefully transfer the hot mixture to a blender and whir until smooth).
Refrigerate until serving or freeze to store longer.** Use as a thicker spread as-is, or thin to a pourable consistency with more water as needed.
*Other sweeteners work as well: try an equal amount of maple syrup or 1/4 c. of honey (since it's a bit sweeter than sugar).**TO FREEZE: fill clean freezer-safe containers (I use half-pint sized) three-quarters full to leave room for expansion, attach lid and freeze 6 months to a year.