Place the washed crabapples in a large pot and add 1 cup of water. Bring to a simmer over medium heat, stirring occasionally, until the crabapples soften and release their juices, about 15 minutes.
Use a potato masher or wooden spoon to gently mash the softened crabapples in the pot, breaking them down into a chunky consistency.
Stir in 1 cup of granulated sugar, 1 tablespoon of lemon juice, and 1/4 teaspoon of ground cinnamon (if using). Continue to simmer over low heat for another 10 minutes, stirring frequently to dissolve the sugar and blend the flavors.
Remove the pot from the heat and let the mixture cool slightly. Pour the sauce through a fine-mesh strainer or food mill into a bowl or jar, using a spoon to press the pulp and extract as much sauce as possible. Discard the seeds and leftover skins.
Transfer the strained sauce to an airtight container and refrigerate until fully chilled. The sauce will thicken as it cools and should have a smooth, spreadable consistency.