Rhubarb: Wash and chop the rhubarb stalks. Place them in a large bowl, add the sugar and mix well. Let it sit for one hour until a syrup forms at the bottom of the bowl. Stir a couple of times during this time.
Apples: Peel and core the apples. Cut them into small pieces.
Combine: Place the rhubarb and all the released liquid, apples, vanilla extract, and lemon juice in a large pot. Stir well to combine.
Cook: Bring to a boil and turn down the heat. Place a lid on top but leave a crack open. Simmer for 8-10 minutes on medium-low heat until the rhubarb breaks down, and the apple pieces are softer. Remove the lid during the last 2-3 minutes of the cooking time.
You can add more sugar at the end if you like the dessert to be sweeter. However, I find it perfect as it is.