Prepare the produce by peeling, coring, and chopping the apples into small chunks. Chop the rhubarb into 1-inch pieces.
Add the chopped rhubarb, apples, sugar, and water to a medium saucepan. Place the pan over medium heat and stir gently to combine.
Bring the mixture to a gentle simmer, heard as a soft bubbling. Cook, stirring occasionally, until the fruit is very tender and starts breaking down, about 15 minutes. The mixture should be fragrant and slightly thickened.
Remove the saucepan from heat and let cool slightly. Then, blend the mixture using an immersion blender or transfer to a blender and pulse until smooth or your desired chunky texture is achieved.
Stir in the vanilla extract if using, then transfer the applesauce to a bowl. Serve warm or chilled, with a bright pink color and a thick, spoonable consistency.