In a medium sized sauce pan add sliced rhubarb, strawberries, and water. Simmer over medium heat until rhubarb has softened.
Blend mixture with an immersion blender and then pour through a fine mesh strainer. You should have about 1 cup of liquid after straining.
Add sugar, egg yolks and 1 cup of strawberry rhubarb mixture back to a clean sauce pan. Whisk together and bring to a simmer until the mixture reaches 165 F.
Remove mixture from the heat and add in butter. Mix until butter has melted.
Transfer curd into a bowl and place a piece of plastic wrap on top to prevent a skin forming. The curd will thicken more as it cools. Store in the refrigerator.