Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a frying pan and fry the chicken breasts on one side only until browned. Remove from the pan with a slotted spoon and reserve on a plate.
Fry the onion and garlic (if used) in the pan, stirring, for 5 minutes or until lightly browned. Add the tomatoes, sun-dried tomatoes and pesto, season with salt and pepper and bring to the boil. Pour into the slow cooker pot, then stir in the stock.
Rinse the brown rice well in a sieve under cold running water, then stir into the slow cooker pot with the wild rice. Arrange the chicken breasts on top of the rice, browned side up, pressing them just below the level of the liquid so that they don't dry out during cooking. Cover with the lid and cook on high for 3–4 hours or until the chicken is cooked and the rice is tender.
Spoon on to serving plates and serve with a rocket salad tossed in an olive oil and lemon dressing