Rip off the skin of the drumsticks. (Tip: I always rip the skin off drumsticks with the aid of a paper towel because it makes the slippery skin easier to grab.) Place in a large bowl and season with salt and pepper.
Trim the fresh lemongrass stalk and grate finely with a microplane rasp grater (my preferred method) or smash the stalk with a meat pounder and cut finely against the grain of the fibers. The finer the dice, the less lemongrass fiber you’ll be picking out of your teeth when you eat the chicken later.
Time to make the marinade! Place the lemongrass, garlic, ginger, coconut milk, fish sauce, coconut aminos, and five spice powder into a high-powered blender and blitz until a smooth sauce is formed.
Pour the marinade onto the chicken and mix well.
Dump the onion into the bottom of the slow cooker and pour the chicken and the marinade on top. Set the slow cooker to low and cook for 4-5 hours.
If you're eating it right away, top it with some fresh chopped scallions! If you're saving it for later, transfer the chicken and the sauce to a storage container to chill in the fridge for a couple days. To reheat the drumsticks, place it in a pot and bring to a simmer.
For an extra creamy sauce, when the stew is done cooking, remove the chicken and put the sauce and onions into a blender. Puree the sauce, and add ¼ cup more of coconut milk into the blender at the final moment. Put the sauce and chicken back into the cooker and keep warm.
