Add the packed brown sugar and stir until dissolved.
Add the cinnamon sticks, cloves, allspice berries, and nutmeg.
Set the slow cooker to low for 5-8 hours or high for a shorter cooking time.
Let the syrup cool, then strain it through a fine mesh strainer into a jar.
Store in the fridge for up to three weeks or freeze for longer storage.
Notes
You can leave out the water and infuse maple syrup or even date syrup. Just leave out the water. If the syrup reduces too thick, add water or non-dairy milk to thin to your desired consistency.