Heat oil in a pan over medium heat; stir in onion and garlic. Cook and stir until onion and garlic have softened, about 5 minutes.
Place chicken breasts in the bottom of a slow cooker; add onion, garlic, vegetable stock, chopped tomatoes, sweet potatoes, quinoa, spring onions, mushrooms, curry powder, oregano, pepper and bay leaf. Mix well and cook on Low until chicken is tender, 7 to 8 hours.
Discard bay leaf. Remove chicken; shred using 2 forks and place back into the stew, mixing well before serving.