Grease the inside of your slow cooker with cooking spray.
In a frying pan, sauté the onions in oil for about 10 minutes.
Add the garlic, tomato purée and chopped anchovy fillets and cook for another 2 minutes. Transfer everything to the slow cooker.
Brown the chicken breasts in the frying pan and place on top of the ingredients already in the slow cooker.
Add the bay leaves, Herbes de Provence, fennel seeds, salt and black pepper.
Add the sliced red peppers and cherry tomatoes.
Pour in the passata and white wine vinegar and stir gently around the chicken.
Cover and cook on HIGH for approximately 2½ hours or LOW for approximately 5 hours.The chicken should be cooked through and tender.
Stir in the olives and freshly chopped parsley.