Add all ingredients to the slow cooker EXCEPT the baby spinach. Add the chicken but leave it whole. Cook for 4 hours on low. Stir occasionally.
At the 4ish hour mark, scoop out all the chicken into a large bowl or plate and set aside to cool slightly.
While the chicken is cooling, use a stick blender to whizz up the liquid in the slow cooker - it will turn into a creamy buttery sauce - yum.
Go back to your chicken and using two forks, pull it apart and shred it into chunks then return it to the creamy buttery sauce, also adding the finely chopped spinach. Stir and cook for a further 30-60 minutes.
During this final cooking time, prepare your sides. Choose from traditional rice, naan bread (this recipe is easy and yeast free https://minimalistbaker.com/fluffy-gluten-free-naan-yeast-free-20-minutes/), GF wraps slice & toasted in oven - brush with olive oil or butter, or I like to make a creamy pumpkin/kumara mash, broccoli, broccolini etc.
Please note this makes a big batch of butter chicken - feeds around 6 people. You could freeze leftovers and have another night or halve the recipe.