Wash and chop the rhubarb into 1-inch pieces. Peel, core, and chop the apples into similar-sized pieces.

In a large, heavy-bottomed pot or Dutch oven, combine the chopped rhubarb, apples, granulated sugar, water, cinnamon, and optional nutmeg.

Bring the mixture to a gentle boil over medium heat, stirring occasionally to prevent sticking. Once boiling, reduce the heat to low, cover the pot, and simmer for 20-25 minutes, or until the fruit is very tender and easily mashed.

Remove the pot from the heat. Carefully mash the fruit mixture to your desired consistency using a potato masher for a chunkier sauce, or an immersion blender for a smoother texture. Alternatively, you can pass it through a food mill for a super smooth consistency.

Stir in the lemon juice. Taste the applesauce and adjust sweetness if necessary, adding more sugar (start with 1-2 tablespoons) if desired.

Allow the applesauce to cool completely before serving or storing. It thickens as it cools.