Melt the butter in a medium skillet over medium heat. Add the onions and saute until soft and translucent. Add the garlic and ginger, cooking for an additional 2 minutes. Transfer to the slow cooker.
Storage Tip: Once the slow-cooker butter chicken has cooled, transfer any leftovers into an airtight container. Afterward, store them in the fridge to enjoy for up to 4 days.