Begin by placing 4 cups of fresh blueberries into your slow cooker. Add 1 cup of light brown sugar, 1/4 cup of honey, the zest and juice of 1 lemon, 1 teaspoon of cinnamon, and 1/4 teaspoon of nutmeg. Stir the mixture until well combined.

Cover the slow cooker and set it to high heat. Allow the mixture to cook for 1 hour.

After an hour, carefully remove the lid and use an immersion blender to puree until smooth. If using a standard blender, transfer in batches. Return to the slow cooker.

Prop the lid open slightly using a wooden spoon, set to low heat, and cook for an additional 4-5 hours, stirring occasionally until thickened.

Ladle the warm blueberry butter into clean glass jars, allow to cool, then seal and store in the refrigerator for up to two months.