Place the pureed strawberries in a slow cooker and set it to low heat.
Cook the berries, stirring occasionally, for 16 to 24 hours. The time depends a lot on how hot your slow cooker gets and whether you're cooking it with or without the lid on the pot. If you’re not going to be around, set a chopstick across the rim of the slow cooker and then put the lid on. This allows the slow cooker to vent a little, but also ends up trapping just enough moisture to prevent the growth of the skin.
So, once your strawberries have cooked down to a dense product that doesn’t have any visible liquid on the surface, it is done. I like to hit it with an immersion blender at the end of cooking, to ensure that it’s perfectly smooth.
Once you like the texture, you add maple syrup to taste. My batches each produced about three half pints, which I sweetened with 1/3 cup of maple syrup. I also included two tablespoons of lemon juice to help keep the color, brighten the flavor, and increase the acid load just a little (strawberries are typically quite high in acid, but maple syrup is low in acid, so a little extra lemon juice makes sure that all is well, safety-wise).
Funnel the hot butter into clean, hot jars. Wipe the rims, apply the lids and rings, and process in a boiling water bath canner for 15 minutes.
When the time is up, remove the lid from the pot and turn off the heat. Let the jars rest in the cooling water for five minutes. When that time is up, remove jars and set them on a folded kitchen towel to cool. When the jars have cooled enough that you can comfortably handle them, check the seals. Sealed jars can be stored at room temperature for up to a year. Any unsealed jars should be refrigerated and used promptly.
