Combine the soy sauce, honey, birch syrup, rice wine vinegar, onions, garlic and ginger in a bowl.
Rub a thin layer of oil around the inside of the slow cooker, add the chicken breasts and pour the mixture over the top. Cook on high for 3-4 hours
Remove the chicken to a bowl and transfer the sauce to a pan. Bring the sauce to a medium heat. Combine the cornflour with 60 ml of water and add this to the sauce. Simmer to thicken and reduce until thick, 5-10 mins. Meanwhile, shred the chicken with a pair of forks. Add the chicken to the sauce when the sauce is thick enough.
Serve as you wish - I went with rice, spring onion sliced over the top and a few sesame seeds sprinkled on, but we used the leftovers for decadent cheesy chicken baguettes!