Start by washing the blueberries thoroughly and removing any stems. In a large mixing bowl, combine the blueberries with brown sugar, honey, lemon zest and juice, cinnamon, and nutmeg. Stir until well combined.

Pour the blueberry mixture into a slow cooker. Spread it evenly, cover, and set it to cook on high for 1 hour.

After 1 hour, uncover and let it cool slightly. Blend the mixture until smooth with an immersion blender or in a regular blender, then return to the slow cooker.

Prop the lid of the slow cooker slightly ajar to allow steam to escape. Set to low and cook for an additional 4 to 5 hours.

Once thickened, jar the blueberry butter using clean glass jars, leaving space at the top, and seal tightly.

Let the jars cool to room temperature before refrigerating. The blueberry butter can be stored in the fridge for up to 2 months.