Use a vegetable peeler to remove the skins from the apples, then chop the peeled apples into uniform small cubes on a cutting board with a chef’s knife.
Add the chopped apples, thawed rhubarb, sugar, and water to a medium saucepan.
4 cups apples, peeled and chopped
Bring the mixture to a gentle simmer over medium heat, stirring occasionally with a wooden spoon or silicone spatula, until the ingredients soften and release steam, about 10 minutes.
Continue cooking, stirring frequently, until the apples break apart and the mixture thickens slightly, about 10 more minutes. The rhubarb should have completely broken down and integrated into the apples, creating a vibrant pink hue.
Remove from heat and let cool slightly before serving. The sauce should be chunky yet smooth with a bright pink color and a balance of tart and sweet flavors.
